Directed Stem Cell Recruitment

ABSTRACT

The invention is directed to methods of inducing cell recruitment and tissue regeneration at a target site in a subject. It is also based, in part, on the discovery that a subject&#39;s own biologic resources and environmental conditions can be used for in situ tissue regeneration and thereby reduce or eliminate the need for donor cell procurement and ex vivo manipulation of such donor cells. Methods are disclosed for recruitment of a subject&#39;s own stem cells to a target region by inducing a sustained positive pressure at a target site, such as the kidney, thereby increasing the number of pluripotent cells capable of differentiating to regenerate the target tissue.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/991,811, filed Jan. 10, 2011, which is a U.S. National 371of PCT/US2009/43446, filed May 11, 2009, which claims benefit toProvisional Application No. 61/051,939, filed May 9, 208, the contentsof which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention concerns tissue engineering and, in particular,methods of recruiting stem cells to a target site.

BACKGROUND OF THE INVENTION

Chronic renal disease is a common condition that elevates the risk ofcomplete renal failure, cardiovascular disease and other complications.The prevalence of chronic renal failure has continuously increased inthe United States in the last decade. Currently, the only treatmentoptions for renal failure are dialysis and transplantation, both ofwhich are associated with considerable cost. Dialysis, usually performed3 times per week, impairs the patient's quality of life and does notreplace renal functions such as synthesis of erythropoietin and vitaminD. Transplantation, on the other hand, can replace all renal functions,but the rising occurrence of end stage renal disease (ESRD) in theUnited States continues to outpace the rate of organ donation, asreflected by the fact that the waiting list continues to grow by 3,000to 4,000 people per year. Furthermore, long-term results of kidneytransplantation remain unsatisfactory, mainly because of chronicrejection and complications associated with immunosuppressivemedications. Therefore, novel therapies for renal failure are needed.

A major goal of tissue engineering and reconstructive surgery is therestoration of structure and function to damaged organs or tissue. Whilethe body's normal reparative processes can heal small, localizedinjuries, a large traumatic injury will often overwhelm the body'snatural restorative systems and result in a deficit of functionalrecovery, despite the use of conventional reconstructive modalities.Organ transplantation has become increasingly commonplace inlife-threatening situations, Such transplantations involve moving awhole or partial donor organ to replace a recipient's damaged or failingone.

However, problems exist when biological material is transferred from oneindividual to another. Organ rejection is a significant risk associatedwith transplantation, even with a good histocompatibility match.Immunosuppressive drugs such as cyclosporin and FK506 are usually givento the patient to prevent rejection. These immunosuppressive drugshowever, have a narrow therapeutic window between adequateimmunosuppression and toxicity. Prolonged immunosuppression can weakenthe immune system, which can lead to a threat of infection. In someinstances, even immunosuppression is not enough to prevent organrejection. Another major problem of transplantation is the availabilityof donor organs. In the United States alone there are about 100,000people on transplant waiting lists, many of whom will die before anorgan becomes available.

Cell-based approaches using tissue engineering and regenerative medicinetechniques have offered new therapeutic opportunities for variouspathologic conditions. During the past decade, a number of differentapproaches for engineering renal tissue have been attempted. The goal ofeach approach was to replace or recover some renal functions. Theserequire several important methodological choices and a number oftechnical difficulties have been encountered. Although the fundamentalprinciples of cell-based therapies have been demonstrated on multipletissue systems clinically, it usually necessitates a donor tissue biopsyand ex vivo cell manipulation prior to implantation in vivo. One of themost critical initial steps is the choice of an appropriate cell source.For typical tissue engineering approaches, cells need to be expanded inlarge quantities, while maintaining uniform activity and remainingpathogen-free. Moreover, the kidney is an extremely complex structurewhich consists of at least 26 terminally differentiated cell types,including tubular epithelial cells, interstitial cells, glomerular cellsand vascular cells.

Most recently, stem cells have been identified as an alternative sourceof cells for tissue regeneration. However, current protocols for the useof stem cells for regeneration typically require harvesting tissues forcell retrieval, isolation of stem cells, in vitro expansion and/ordifferentiation of the isolated stem cells, and reimplantation of themanipulated cells into specific tissue sites in vivo for restoration oforgan/tissue function.

There exists a need for better methods of tissue regeneration. Inparticular, new methods of restoring structure and/or function todamaged or failing body structures would satisfy a long-felt therapeuticneed.

SUMMARY OF THE INVENTION

The invention is based, in part, on the discovery that a subject's ownbiologic resources and environmental conditions can be used for in situtissue regeneration and thereby reduce or eliminate the need for donorcell procurement and ex vivo manipulation of such donor cells. Methodsand compositions are disclosed for recruitment of a subject's own stemcells to a target region by inducing a sustained positive pressure at atarget site.

This invention reduces or eliminates the need for laborious stem cellisolation and ex vivo cell culturing by directly recruiting a stem cellpopulation into target specific sites for regeneration in vivo. This ispossible due to the nature of host cellular responses to environmentchanges. In one aspect of the invention, the use of a biocompatiblesubstance as an implant can be used to recruit pluripotent cells to atarget site within a body structure and induce tissue regenerationwithin the body structure in vivo using a “directed cell recruitment”technique. Moreover, a method can be directed to inducing cellrecruitment to a target site by delivering a biocompatible substance toa target site within a body structure to produce positive pressurewithin the target site and maintaining the positive pressure, such thatpluripotent cells are recruited to the target site. Another aspect canbe directed to a method of inducing in situ tissue regeneration at atarget site of a subject by delivering a biocompatible substance to thetarget site to produce a positive pressure, maintaining the positivepressure for a period of time such that the number of pluripotent cellsis increased at the target site, delivering at least one adjuvant to thetarget site and promoting differentiation of the pluripotent cells toregenerate the tissue.

To demonstrate this technique, several injectable scaffolds have beenused to show that cell recruitment assisted regeneration is possible ina rodent kidney model. Kidney structures, including glomeruli andtubules, were formed within the injected gel scaffolds in the renalparenchyma 1 week after injection and continued to mature with time.

Current treatment options for renal failure are extremely limited andonly renal transplantation can restore kidney function. This inventioncan provide an alternative treatment modality for patients with renalfailure. The outcome of kidney tissue regeneration suggests that thistechnology can be applied in other organ/tissue systems. One aspect ofthe invention can be directed to kidney regeneration by injecting asubstantially cell-free biocompatible substance into a target site in akidney, such that the biocompatible substance creates a hyperbaricenvironment at the target site, maintaining the positive pressure for atleast one hour to recruit pluripotent cells, and inducingdifferentiation of the pluripotent cells to produce new glomerulistructures.

As shown in the examples, collagen based gel scaffolds, and otherbiomaterials, including collagen type I, collagen based kidney tissuegel matrix, synthetic gel matrix and keratin based gel matrix weretested in regeneration of kidney tissues. The results of all injectionsdemonstrated similar findings with formation of glomerular and tubularstructures. Thus, stem cells or progenitor cells can be recruited totarget specific sites, and corresponding cells and tissues can beformed.

The recruited stem cells can be differentiated into target tissues forregeneration. Various tissues and organ systems can be regenerated usingthe methods of the present invention, including, but not limited to,kidney, liver, spleen, pancreas, muscle, heart, skin, lung, cartilage,spinal cord, bone, spleen, bladder, ureter, urethra, intestine, thymus,and thyroid.

In another aspect, the invention discloses a method inducing cellrecruitment to a target site of a subject, preventing inflammatorycells, and/or reducing collagen deposition at and around the site. Thebiocompatible substance can also comprise at least one adjuvantdelivered to the target site, and/or incorporated into the biocompatiblesubstance. The adjuvant can be incorporated in or on the biocompatiblesubstance and can be delivered separate from the biocompatiblesubstance. The adjuvant can be selected from growth factors, cytokines,enzymes, collagen reducing agents, antibiotics and anti-inflammatoryagents. The adjuvants can also be, for example, anti-inflammatory agentsand/or collagen synthetase inhibitors. In preferred embodiments, theanti-inflammatory agents counteract or suppress the inflammatoryprocess. In some embodiments, the anti-inflammatory agent is a collagensynthetase inhibitor. Anti-inflammatory agents can be steroidal ornon-steroidal. Non-limiting examples of anti-inflammatory agentsinclude, corticosteroids, dexamethasone, rapamycin, paclitaxel, ABT-578,everolimus, taxol, steroidal anti-inflammatory agents, non-steroidalanti-inflammatory agents, hemostatic agents; antimicrobial agents;antibiotics; antifungals; antiprotozoals; antivirals; antimicrobialmetals; hemostatic and/or vasoconstricting agents; pseudoephedrine;xylometazoline; oxymetazoline; phenylephrine; epinephrine; cocaine;local anesthetic agents; lidocaine; cocaine; bupivacaine; hormones;hormonally active agents; agents that enhance potency; substances thatdissolve, degrade, cut, break, weaken, soften, modify or remodelconnective tissue or other tissue; enzymes; trypsin; EDTA; trypsincombined with EDTA; hyaluronidase; tosyllysylchloromethane (TLCM);chemotherapeutic or antineoplastic agents; substances that preventadhesion formation; hyaluronic acid gel. Non-limiting examples ofcollagen synthetase inhibitors include collagenase, halofuginone, propylhydroxylase, c-proteinase inhibitor, and metalloproteinase. In apreferred embodiment, the adjuvant can be collagen synthetase inhibitor,such as collagenase. In another preferred embodiment, the adjuvant canbe an anti-inflammatory agent, such as dexamethasone.

The examples were performed in the subcutaneous tissue system in orderto show that stem cells can be directed into specific target sites. Thissystem can be applied in other tissues or organ systems. Syntheticmaterials were used to demonstrate that no biological factors wereneeded to recruit stem cells to the target site. In some embodiments ofthe invention, biological scaffolds incorporating cell and/or tissuedifferentiation factors can be combined to regenerate tissues in vivo.The biocompatible substance can be a natural or synthetic polymer. Thebiocompatible substance can take any form, such as implantable orinjectable biomaterials, such as a hydrogel. In one embodiment, thebiocompatible substance is a substantially cell-free, injectablebiocompatible polymeric substance. Preferable, the biocompatiblesubstance is sufficiently porous to allow cell infiltration. Also, thebiocompatible substance can provide a scaffold for attachment of thepluripotent cells. In a preferred embodiment, the biocompatiblesubstance can be a hydrogel formulated to expand by water absorptionfollowing implantation at the target site to provide a sustainedpositive pressure. In a preferred embodiment, the biocompatiblesubstance is collagen. In another preferred embodiment, thebiocompatible substance can comprise a solution having a collagenconcentration from about 1 mg/ML to about 30 mg/ML. More preferrably,the collagen solution thermogels at about 37° C.

In another aspect, the invention discloses target specific gelscaffolds. These target specific gel scaffolds can maximize the tissueregenerative capacity. Incorporation of factors, such as growth factors,extracellular matrix (ECM) proteins and bioactive molecules into the gelscaffold system can enhance tissue formation. This gel system can beused as a preventive measure in subjects with high risk of organ ortissue failure. The scaffolds can take various forms, depending on thetissue and its use. The biocompatible substance can be delivered intothe target site via injection, surgically placing the substance into thetarget site, or by using a catheter.

In another aspect, the invention discloses tissue and/or organ specificscaffolds for regenerating, restoring, and/or augmenting variousdiseased tissues and organs. The tissue and/or organ specific scaffoldcan be used, for example, in subjects with or at risk for organ failure,or as preventive measures for organ maintenance.

In yet another aspect, the invention provides a method of inducing cellrecruitment to a target site of a subject comprising delivering abiocompatible substance into a target site capable of producing asustained increase in positive pressure within the target site, andmaintaining the increase in positive pressure at the target site for aperiod of time, whereby the number of pluripotent cells at the targetsite is increased. The positive pressure is capable of being sustainedbetween about 5 cmH₂O to about 70 cmH₂O, or about 5 cmH₂O to about 50cmH₂O, or about 10 cmH₂O to about 40 cmH₂O, or about 10 cmH₂O to about30 cmH₂O, or about 15 cmH₂O to about 35 cmH₂O, or about 20 cmH₂O toabout 30 cmH₂O after delivery at the target site. In some embodiments,the biocompatible substance has a viscosity between about 5 cP to about1×10⁸ cP, or about 5 cP to about 1×10⁶ cP, or about 5 cP to about 1×10⁵cP, or about 5 cP to about 1×10⁴ cP, or about 10 cP to about 1×10³ cP,or about 6 cP to about 9500 cP at 25° C.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows histological evaluations of the implanted cell-freescaffolds. FIG. 1A shows the number of recruiting cells. Theinfiltrating cells gradually increased in number up to 3 weeks afterimplantation. FIG. 1B shows DNA content in the implanted scaffold. DNAcontent indicates cellularity within the implanted scaffold. DNA contentgradually increased up to 3 weeks after implantation.

FIG. 2 shows histological evaluation of the implanted scaffolds. Contentof soluble collagen. Collagen deposition within the implants graduallyincreased over time.

FIGS. 3A-3H are FACS analysis graphs of infiltrating cells characterizedfor expression of various markers. FIG. 3A is a FACS graph forexpression of Sca-1; FIG. 3B is a FACS graph for expression of Flk-1;FIG. 3C is a FACS graph for expression of CD44 (C); FIG. 3D is a FACSgraph for expression of CD45; 3E is a FACS graph for expression of CD31;FIG. 3F is a FACS graph for expression of CD34, FIG. 3G is a FACS graphfor expression of CD90, and FIG. 3H is a FACS graph for expression ofCD117. Isotype-matching IgG and a FITC-labeled secondary antibody wereused to determine nonspecific signals.

FIG. 4 shows calcification of infiltrating cells under osteogenicconditions. Mineralization of cells was quantified using Alizarin redAssay for calcium. Numbers represent the averages of calcium depositionin culture well. Osteogenic induced cells showed a significant increaseof calcium deposition after 16 days of culture.

FIG. 5 shows dexamethanson-incorporated PGA scaffolds; (A) DNA contentin the implanted scaffolds (P<0.05). (B) Contents of soluble collagensin the implanted scaffolds (P<0.05).

FIG. 6 is a graph showing the numbers of glomeruli in the injured areaat week 2 after hydrogel injection (based on ×100 magnification of PCNAstaining, *P<0.05, **area=0.57 mm²).

FIG. 7 is a graph showing the number of glomeruli in the injured area atweek 22 after hydrogel injection (based on ×100 magnification of PCNAstaining, *P<0.05, **area=0.57 mm²).

FIG. 8A shows a schematic of the CD1 mouse model kidney. FIG. 8B shows aschematic of the space created in the kidney by a 2 mm biopsy punch.

FIG. 9A is a graph comparing the pressure within the kidney followinginjection of 2 mg/ml collagen solution versus time and FIG. 9B is asimilar graph comparing pressure within the kidney following injectionof saline versus time.

FIG. 10 is a graph that depicts the number of PCNA-positive cells in thenormal and injection regions after 2 week injection (*P<0.01).

FIG. 11 is a graph that depicts the quantitative analysis of number ofglomeruli per area in the normal CD1 mouse kidneys after injection(*P<0.05).

FIGS. 12A-B are graphs that show the quantitative analysis of number ofglomeruli in the injection regions of the renal ischemia/reperfusionrats; FIG. 12A shows the number of glomeruli per whole kidney area andFIG. 12B shows the number of glomeruli per mm2 (*P<0.05).

FIG. 13 is a graph that shows the blood serum analysis of creatininelevels for functional evaluation of the ischemic injured kidneys.

FIGS. 14A and 14B are graphs showing measurements of a partial pressurein the injection regions after injection; FIG. 14A shows partialpressure measurements in a mouse kidney and FIG. 14B shows partialpressure measurements in a rat kidney.

FIG. 15 shows measurement of a partial pressure in the injection regionsafter collagen injection with different concentrations in mice.

FIG. 16 shows multiple injections with different time points. Number ofglomeruli of the whole kidney after collagen gel injection with multipletime points (P<0.05).

DETAILED DESCRIPTION

The invention is directed to methods of inducing cell recruitment andtissue regeneration at a target site in a subject. It is also based, inpart, on the discovery that a subject's own biologic resources andenvironmental conditions can be used for in situ tissue regeneration andthereby reduce or eliminate the need for donor cell procurement and exvivo manipulation of such donor cells. Methods are disclosed forrecruitment of a subject's own stem cells to a target region by inducinga sustained positive pressure at a target site, such as the kidney,thereby increasing the number of pluripotent cells capable ofdifferentiating to regenerate the target tissue.

As shown in the examples, a common biomaterial was implanted into miceand the infiltrating cells were characterized to determine theirregenerative potential. In contrast to prior belief, the host cellinfiltrates are not entirely comprised of inflammatory andfibroblast-like cells. The normal inflammatory process can be altered byincorporating anti-inflammatory agents that influence the formation ofscar tissue. In addition, the infiltrating cells are capable ofdifferentiating into multiple cell lineages, including osteogenic,myogenic, adipogenic and endothelial, if appropriate conditions areprovided. The examples show that it is possible to recruit apredominance of cells with multilineage potential into a biomaterialscaffold. The infiltrate can be enriched with such cell types and theirfate can be controlled, provided the proper substrate-mediated signalingis imparted into the scaffold for in situ tissue regeneration.

A typical clinical solution for repairing large tissue defects is torestore some level of function through the use of an implant. Surgicalimplants can consist of autografts (e.g. tissue flaps), allografts (e.g.cadaveric tissues), and a multitude of synthetic and naturally derivedbiomaterials. Although many of these modalities are able to achieve thetargeted goals, various limitations remain a challenge, which rangesfrom donor tissue unavailability to procedure related complications.Recent advances in tissue engineering and regenerative medicine haveoffered new opportunities for treatment of tissue and organ deficits.Various cell and biomaterial related technologies have allowed for thedevelopment of biological substitutes which are designed to restore andmaintain normal tissue and organ function (Atala A. Recent developmentsin tissue engineering and regenerative medicine. Curr Opin Pediatr 2006;18:167-171; Williams D J, Sebastine I M. Tissue engineering andregenerative medicine: manufacturing challenges. IEE Proc Nanobiotechnol2005; 152:207-210).

Classic tissue engineering approaches have been employed to overcome thepresent challenges by implementing biomaterial scaffolds that arepre-seeded with desirable cell types to generate functional tissues thatprogressively mature when introduced in vivo. Using this strategy, manypathologic tissue conditions such as urethral stricture, bladderdysfunction, vascular grafts, and osteoarthritic cartilage defect, havebeen treated clinically. (El-Kassaby A W, Retik A B, Yoo J J, Atala A.Urethral stricture repair with an off-the-shelf collagen matrix. J Urol2003; 169:170-173; Atala A, Bauer S B, Soker S, Yoo J J, Retik A B.Tissue-engineered autologous bladders for patients needing cystoplasty.Lancet 2006; 367:1241-1246; Shin′oka T, Matsumura G, Hibino N, Naito Y,Watanabe M, Konuma T, Sakamoto T, Nagatsu M, Kurosawa H. Midtermclinical result of tissue-engineered vascular autografts seeded withautologous bone marrow cells. J Thome Cardiovasc Surg 2005;129:1330-1338; Ossendorf C, Kaps C, Kreuz P C, Burmester G R, SittingerM, Erggelet C. Treatment of posttraumatic and focal osteoarthriticcartilage defects of the knee with autologous polymer-basedthree-dimensional chondrocyte grafts: 2-year clinical results. ArthritisRes Ther 2007; 9:R41.). Although the principle of this technology hasbeen demonstrated in various preclinical and clinical studies indifferent tissue systems, the approach has typically necessitated adonor tissue biopsy, followed by cell isolation and expansion whichrequires extensive cell manipulation prior to implantation in vivo.(Langer R, Vacanti J P. Tissue engineering. Science 1993; 260:920-926.)In instances where donor cells are unavailable due to extensive tissuedamage, stem and progenitor cells have been considered as alternate cellsources. However, the use of these cells requires a similar approachincluding ex vivo procedures such as expansion and/or differentiationinto specific cell lineages for cell-based therapies. (Guillot P V, CuiW, Fisk N M, Polak D J. Stem cell differentiation and expansion forclinical applications of tissue engineering. J Cell Mol Med 2007; 11:935-944.)

Simplifying these processes by eliminating a tissue biopsy and in vitrocell isolation, expansion and differentiation steps would provide a moreefficient means of developing biological substitutes for functionaltissue restoration in vivo. This proposition may be possible by tappingthe body's innate regenerative systems which have all the biologicalresources necessary for tissue regeneration. It is widely accepted thatalmost every tissue in the body contains some type of stem or progenitorcells, including brain, liver, circulating blood, heart, skin, fat andmuscle. (Gage F H. Mammalian neural stem cells. Science 2000;287:1433-1438; Zhang Y, Bai X F, Huang C X. Hepatic stem cells:existence and origin. World J Gastroenterol 2003; 9:201-204; Asahara T,Murohara T, Sullivan A, Silver M, van der Z R, Li T, Witzenbichler B,Schatteman G, Isner J M. Isolation of putative progenitor endothelialcells for angiogenesis. Science 1997; 275:964-967; Pfister O, Mouquet F,Jain M, Summer R, Helmes M, Fine A, Colucci W S, Liao R. CD31− but notCD31+ cardiac side population cells exhibit functional cardiomyogenicdifferentiation. Circ Res 2005; 97:52-61; Bartsch G, Yoo J J, De C P,Siddiqui M M, Schuch G, Pohl H G, Fuhr J, Perin L, Soker S, Atala A.Propagation, expansion, and multilineage differentiation of humansomatic stem cells from dermal progenitors. Stem Cells Dev 2005;14:337-348; De Ugarte D A, Ashjian P H, Elbarbary A, Hedrick M H. Futureof fat as raw material for tissue regeneration. Ann Plast Surg 2003;50:215-219; Deasy B M, Huard J. Gene therapy and tissue engineeringbased on muscle-derived stem cells. Curr Opin Mol Ther 2002; 4:382-389.)It would seem that these cells are part of underlying regenerativemachinery that is responsible for daily maintenance activities,including repair of normal tissue wear and tear, as well as small,non-life threatening types of injuries. However, when extensive tissuedamage occurs and large tissue defects are present, the regenerativeresponse is overwhelmed and an immune-based reparative response takesover to maintain some level of function. (Cotran R Z, Kumar V, Robbins SL. Robbins Pathologic Basis of Disease. Philadelphia, Pa., W B Saunders,1999) While the immediate problem may be mitigated by these reparativeprocesses, responses such as inflammation which results in uncontrolledcollagen deposition and fibrosis are undesirable because they can leadto further complications and severe deficits in tissue and organfunctionality. (Morehead J M, Holt G R. Soft-tissue response tosynthetic biomaterials. Otolaryngol Clin North Am 1994; 27:195-201; TangL, Eaton J W. Inflammatory responses to biomaterials. Am J Clin Pathol1995; 103:466-471; Tang L, Ugarova T P, Plow E F, Eaton J W. Moleculardeterminants of acute inflammatory responses to biomaterials. J ClinInvest 1996; 97:1329-1334; Mikos A G, McIntire L V, Anderson J M,Babensee J E. Host response to tissue engineered devices. Adv Drug DelivRev 1998; 33:111-139; Hu W J, Eaton J W, Ugarova T P, Tang L. Molecularbasis of biomaterial-mediated foreign body reactions. Blood 2001;98:1231-1238.)

Toward this goal, it can be established that the host cell infiltratethat accompanies every foreign body reaction has at least some capacityfor regeneration. However, this notion conflicts with the current dogmathat the host cell infiltrate is comprised primarily of immune andfibroblast-like cells, which are believed to be responsible for theformation of scar tissue. Remarkably, this conclusion has been reachedin the absence of definitive experiments that incorporate the use ofunique, specific immunohistochemical markers capable of identifying thecell types in retrieved biomaterial implants. A first step in reversingthis historical tenet is to demonstrate the host cell infiltratecontains cells other than fibroblast-like cells. As shown in theexamples, an animal model was used to initiate cell infiltration into animplanted biomaterial, followed by extensive and definitivecharacterization of that cell infiltrate. The examples demonstrate theuse of infiltrating host cells as an in situ source for tissueregeneration, which can be used for donor cell procurement andsubsequent in vitro cell manipulation.

A normal human body possesses biologic resources and an idealenvironment for recovery from tissue damage due to various insults. Thisis usually achieved through normal wound healing process which isinitiated by inflammatory and immunologic responses. While small andminor wounds can be repaired through this process without causingfunctional tissue abnormalities, large tissue defects due to extensivetissue trauma usually results in a functional deficit. In suchinstances, various reconstructive measures are necessary to restorefunctionality of the affected tissues and organs. Cell based approachesusing tissue engineering and regenerative medicine techniques have beenemployed to repair defects for partial or full restoration of affectedtissue function. However-, these approaches usually require cell/tissuebiopsy and extensive cell manipulation in vitro prior to implantation invivo.

Functional recovery in acute renal failure has been demonstratedrecently. Several researchers have reported the existence of renalstem/progenitor cells which can contribute to regeneration and repair inthe kidney. Several genes expressed during embryonic development aredownregulated in mature kidney tissue, but are expressed again duringrecovery after renal injury. One such factor is paired box gene 2(PAX-2). PAX-2 belongs to a family of transcription factors, and isrequired for development and proliferation of renal tubules. Renalprogenitor cells expressing CD24, CD133, and PAX-2 have been identifiedat the tubular and glomerular levels and can regenerate tubular cells inan animal model of acute renal failure. It has been demonstrated thatthe presence of a resident population of stem cells expressing CD 133and PAX-2 markers in adult normal human kidney are capable of expansionand, potentially, self-renewal. In addition, bone marrow-derived cellsrepresent potential source of cells that can regenerate renal tubules.The presence of an underlying regenerative mechanism in the form oftissue specific stem and progenitor cells suggests that there can be anopportunity to bias the host response toward repair of renal injury.

The invention discloses methods of utilizing the body's biologic andenvironmental resources in situ for tissue regeneration. The examplesdemonstrate whether implantation of cell-free biomaterials can recruithost cells that can participate in the tissue repairing process. A largeproportion of infiltrated host cells within the biomaterial implants wasdemonstrated to have multilineage potential (i.e. 85% by flow cytometryanalysis). Using standard protocols for immunocytochemistry and flowcytometry, we confirmed that these cells are able to express ahematopoietic stem cell marker, Sca-1. However, these cells did notexpress endothelial progenitor markers such as Flk-1, CD31, CD34, orCD45; nor did they express mesenchymal stem cell markers, includingCD44, CD45, CD90, or CD117. Sca-1 expression persisted for over fiveconsecutive subcultures. Moreover, differentiation experimentsdemonstrated that these cells are able to transform into osteogenic,endothelial, adipogenic, and myogenic lineages. These results indicatethat many of the cells that are mobilized into a biomaterial aremultipotent, and given an optimal environment, they can differentiateinto specific cell types functional for regenerating the implant site.

To examine the effects of environmental cues on the infiltrating cellsand to determine whether normal host response can be altered,dexamethasone, a well-known anti-inflammatory corticosteroid, wasincorporated into the biomaterial implants. This environment allowedhost cell infiltration but delayed collagen deposition within the targetsites over time (FIG. 5). This finding demonstrates that removal ofpro-inflammatory signals can allow the multipotent cells present in thescaffold to initiate a regenerative process. This experiment indicatesthat environmental cues can be controlled and further suggests that hostcells can be utilized and manipulated in situ for target tissueregeneration.

I. Definitions

So that the invention may more readily be understood, definitions knownby those skilled in the art are described below:

The phrases “augmenting organ function” or “augmenting function of anorgan” as used herein refers to increasing, enhancing or improving thefunction of an organ or body structure that is operating at less thanoptimum capacity. The term is used to refer to a gain in function sothat the organ or structure is operating at a physiologically acceptablecapacity for that subject. For example, the physiological acceptablecapacity for an organ from a child, e.g., a kidney or heart, would bedifferent from the physiological acceptable capacity of an adult, or anelderly patient. The entire organ or part of the organ can be augmented.Preferably the augmentation results in an organ with the samephysiological response as a native organ. In a preferred embodiment, anorgan is augmented in capacity when it is functioning to at least at 10%of its natural capacity.

The phrase “biocompatible substance” and term “biomaterial” are usedinterchangeably and refer to a material that is suitable forimplantation or injection into a subject. A biocompatible substance doesnot cause toxic or injurious effects once implanted in the subject. Inone embodiment, the biocompatible substrate is a polymer with a surfacethat can be shaped into the desired structure that requires repairing orreplacing. The polymer can also be shaped into a part of a bodystructure that requires repairing or replacing. In another embodiment,the biocompatible substrate can be injected into a subject at a targetsite.

As used herein, the term “stem cell” refers to a master cell that canreproduce indefinitely to form the specialized cells of tissues andorgans. The terms “stem cell” and “pluripotent cell” are usedinterchangeably herein. A stem cell (or pluripotent) can divide toproduce two daughter stem cells, or one daughter stem cell and oneprogenitor (“transit”) cell, which then proliferates into the tissue'smature, fully formed cells. The “stem cell” used herein includes“progenitor cells” unless otherwise noted. The term “pluripotential”,“pluripotential for differentiation” or “pluripotent” can also refer toa cell that is positive for one or more of the pluripotent markers suchas but are not limited to Oct-4, Nanog, and Sox-2 and the cell has thepotential to differentiate to at least a subset of the mammalian body'sapproximately 260 cell types upon appropriate stimulations such as bythe appropriate growth factors.

The term “subject,” as used herein, refers to any living organismcapable of eliciting an immune response. The term subject includes, butis not limited to, humans, nonhuman primates such as chimpanzees andother apes and monkey species; farm animals such as cattle, sheep, pigs,goats and horses; domestic mammals such as dogs and cats; laboratoryanimals including rodents such as mice, rats and guinea pigs, and thelike. The term does not denote a particular age or gender. Thus, adultand newborn subjects, as well as fetuses, whether male or female, areintended to be covered.

The term “target site” as used herein refers to region in the organ orbody structure that requires augmentation. The target site can be asingle region in the organ, or can be multiple regions in the organ. Theentire organ or part of the organ can be augmented. Preferably theaugmentation results in an organ with the same physiological response asa normal organ. The entire organ can be augmented by placing a pluralityof biomatrices at suitable distances along the entire organ, e.g., alongthe entire longitudinal section of a kidney. Alternatively, part of theorgan can be augmented by placing at least one biomatrix in one targetsite of the organ, e.g., the top of the kidney.

II. Biomaterials

The phrase “biocompatible substance” and term “biomaterial” are usedinterchangeably and refer to a material that is suitable forimplantation or injection into a subject. A biocompatible substance doesnot cause toxic or injurious effects once implanted in the subject. Inone embodiment, the biocompatible substrate is a polymer with a surfacethat can be shaped into the desired structure that requires repairing orreplacing. The polymer can also be shaped into a part of a structurethat requires repairing or replacing. In another embodiment, thebiocompatible substrate can be injected into a subject at a target site.Preferably, the biocompatible substance can be an attachment structurefor cells promoting cell recruitment and tissue regeneration.

Preferably, the biocompatible substance can create a hyperbaricenvironment at the target site. The biocompatible substance can alsoprovide sustained positive pressure between about 5 cmH₂O to about 70cmH₂O, or about 5 cmH₂O to about 50 cmH₂O, or about 10 cmH₂O to about 40cmH₂O, or about 10 cmH₂O to about 30 cmH₂O, or about 15 cmH₂O to about35 cmH₂O, or about 20 cmH₂O to about 30 cmH₂O, for a period of time, andhas a viscosity greater than that of water. For example, thebiocompatible substance is capable of providing sustained positivepressure for longer than about 1 hour, longer than about 12 hours,longer than about 1 day, longer than about 3 days, longer than about 5days, longer than about 10 days, longer than about two weeks. Theexamples demonstrate that created space is insufficient for tissueregeneration indicating that pressure can influence cell recruitment,preferably stem cell recruitment, of native cells in the tissue into thetarget area to initiate the regenerative process. In some embodiments,the biocompatible substance has a viscosity between about 5 cP to about1×10⁸ cP, or about 5 cP to about 1×10⁶ cP, or about 5 cP to about 1×10⁵cP, or about 5 cP to about 1×10⁴ cP, or about 10 cP to about 1×10³ cP,or about 6 cP to about 9500 cP at 25° C.

Natural or synthetic polymers can be used as the biocompatiblesubstance. Synthetic polymers that can be used to form the microspheresinclude bioerodible polymers such as poly(lactide) (PLA), poly(glycolicacid) (PGA), poly(lactide-co-glycolide) (PLGA), poly(caprolactone),polycarbonates, polyamides, polyanhydrides, polyamino acids, polyorthoesters, polyacetals, polycyanoacrylates and degradable polyurethanes,and non-erodible polymers such as polyacrylates, ethylene-vinyl acetatepolymers and other acyl substituted cellulose acetates and derivativesthereof, non-erodible polyurethanes, polystyrenes, polyvinyl chloride,polyvinyl fluoride, poly(vinylimidazole), chlorosulphonated polyolefins,and polyethylene oxide. Examples of natural polymers include proteinssuch as collagen, albumin, synthetic polyamino acids, and prolamines,and polysaccharides such as alginate, heparin, and other naturallyoccurring biodegradable polymers of sugar units.

In some embodiments, the biocompatible substance can have incorporatedbiomaterials, such as polymers used to encapsulate growth factors,drugs, or other agents which can be released at the target site. Thenormally charged outer layer of the microcapsules can be covered bywater soluble non-ionic polymers such as poly(ethylene oxide) (PEO)which act to shield the charge. These polymers are grafted to thepolycationic polymers, such as poly-L-lysine (PLL) molecules used as atleast one of the layers of the microcapsule, such that they create anon-ionic barrier between the outer layer of the microcapsule (made ofessentially either polycationic polymers, such as PLL, or polyanionicpolymers, such as alginate) and the target tissue.

PLA, PGA and PLA/PGA copolymers are particularly useful for formingmicrospheres. PLA polymers are usually prepared from the cyclic estersof lactic acids. Both L(+) and D(−) forms of lactic acid can be used toprepare the PLA polymers, as well as the optically inactive DL-lacticacid mixture of D(−) and L(+) lactic acids. Methods of preparingpolylactides are well documented in the patent literature. The followingU.S. patents, the teachings of which are hereby incorporated byreference, describe in detail suitable polylactides, their propertiesand their preparation: U.S. Pat. No. 1,995,970 to Dorough; U.S. Pat. No.2,703,316 to Schneider; U.S. Pat. No. 2,758,987 to Salzberg; U.S. Pat.No. 2,951,828 to Zeile; U.S. Pat. No. 2,676,945 to Higgins; and U.S.Pat. Nos. 2,683,136; 3,531,561 to Trehu.

By altering the properties of the polymer and the properties of thedosage form, one can control the contribution of each of these releasemechanisms and alter the release rate of factors. Slowly erodingpolymers such as poly(L-lactide) or high molecular weightpoly(lactide-co-glycolide) with low glycolide compositions will causethe release to become diffusion controlled. Increasing the glycolidecomposition and decreasing the molecular weight enhances both wateruptake and the hydrolysis of the polymer and adds an erosion componentto the release kinetics. In a preferred embodiment, the biocompatiblesubstance comprises alginate-PLL capsules.

A biocompatible substance can also be biodegradable. Biodegradablerefers to material that can be absorbed or degraded in a patient's body.Representative materials for forming the biodegradable structure includenatural or synthetic polymers, such as, for example, collagen, poly(alpha esters) such as poly (lactate acid), poly (glycolic acid) (PGA),polyorthoesters and polyanhydrides and their copolymers, which degradedby hydrolysis at a controlled rate and are reabsorbed. These materialsprovide the maximum control of degradability, manageability, size andconfiguration. Preferred biodegradable polymer materials includepolyglycolic acid and polygalactin, developed as absorbable syntheticsuture material. Polyglycolic acid and polygalactin fibers may be usedas supplied by the manufacturer. Other biodegradable materials includecellulose ether, cellulose, cellulosic ester, fluorinated polyethylene,phenolic polymer,poly-4-methylpentene, polyacrylonitrile, polyamide,polyamideimide, polyacrylate, polybenzoxazole, polycarbonate,polycyanoarylether, polyester, polyestercarbonate, polyether,polyetheretherketone, polyetherimide, polyetherketone, polyethersulfone,polyethylene, polyfluoroolefin, polyimide, polyolefin, polyoxadiazole,polyphenylene oxide, polyphenylene sulfide, polypropylene, polystyrene,polysulfide, polysulfone, polytetrafluoroethylene, polythioether,polytriazole, polyurethane, poly vinyl, polyvinylidene fluoride,regenerated cellulose, silicone, urea-formaldehyde, or copolymers orphysical blends of these materials. The material can be impregnated withsuitable antimicrobial agents and can be colored by a color additive toimprove visibility and to aid in surgical procedures.

The biocompatible substance can be non-biodegradable such that a growthfactor or agent encapsulated in a biodegradable substance can besecreted through the biocompatible substance in a controlled releasemanner while the biocompatible substance remains. Semipermeablemicrocapsules can be produced through interfacial polymerization asdescribed in U.S. Pat. No. 4,251,387. In a preferred embodiment,alginate-PLL capsules are used. Microencapsulation generally involvesthree steps: (a) generating microcapsules (e.g., by forming droplets ofliquid alginate followed by exposure to a solution of calcium chlorideto form a solid gel), (b) coating the resulting gelled spheres withadditional outer coatings (e.g., outer coatings comprising polylysineand/or polyornithine) to form a semipermeable membrane; and (c)liquefying the original core gel (e.g., by chelation using a solution ofsodium citrate). The three steps are typically separated by washings innormal saline.

Alginates are linear polymers of mannuronic and guluronic acid residues.Monovalent cation alginate salts, e.g., Na-alginate, are generallysoluble. Divalent cations such as Ca²⁺, Ba²⁺ or Sr²⁺ tend to interactwith guluronate, providing crosslinking and forming stable alginategels. Alginate encapsulation techniques typically take advantage of thegelling of alginate in the presence of divalent cation solutions.Alginate encapsulation generally involves encapsulation in a solution ofa monovalent cation alginate salt to generate droplets of this solution,and contacting the droplets with a solution of divalent cations. Thedivalent cations interact with the alginate at the phase transitionbetween the droplet and the divalent cation solution, resulting in theformation of a stable alginate gel matrix being formed. A variation ofthis technique is reported in U.S. Pat. No. 5,738,876, wherein the cellis suffused with a solution of multivalent ions (e.g., divalent cations)and then suspended in a solution of gelling polymer (e.g., alginate), toprovide a coating of the polymer. In some embodiments, the biocompatiblesubstance can be a hydrogel.

While it is well known that implants can become populated with hostcells that can result in scar tissue, the cell types have previouslybeen assumed to be inflammatory and fibroblastic, as indirect evidence(i.e. the presence of collagen) has suggested that fibroblasts are thepredominant cell population present after the initial inflammation hassubsided. As shown in the examples, a simple approach was used toaddress this dogma by using PGA nonwoven implants. This polymer has beenwidely used in tissue engineering and regenerative medicine as abiocompatible, biodegradable, and implantable biomaterial. PGA can beused as a synthetic biomaterial because the host immune response to PGAscaffolds has been well characterized by many investigators in amultitude of animal models, demonstrating that it induces a classicforeign body reaction. (Athanasiou K A, Niederauer G G, Agrawal C M.Sterilization, toxicity, biocompatibility and clinical applications ofpolylactic acid/polyglycolic acidcopolymers. Biomaterials 1996;17:93-102.). The PGA mesh used in this study is highly porous and isdesigned to increase diffusion and accommodate host cell infiltrates.Experimental results show that the number of host cells continue toincrease up to 3 weeks after implantation and begin to decreasethereafter as collagen accumulates and fills the pores of PGA mesh. Thisis consistent with normal inflammatory response seen in many tissuesystems.

This invention demonstrates that host cell infiltrates into abiomaterial implant are not entirely comprised of inflammatory andfibroblast-like cells. In the Examples it is shown that cells expressinghematopoietic markers are mobilized into the biomaterial and that thesecells are capable of differentiating into multiple cell lineages ifappropriate conditions are provided. In other aspect, the inventionprovides a tissue-specific cell-free biomaterial that can be universallyapplied to any patient, without the need for ex vivo cell manipulation.Ideally, the patient's body would provide both the source of cells andthe environment for terminal differentiation, provided the appropriatecues can be mediated through the biomaterial. In contrast to currentmodalities that focus on in vitro manipulation of cells, the inventionprovides methods for controlling tissue morphogenesis in vivo byproviding the appropriate cues to infiltrating multipoint cells, leadingto the production of functional tissues in situ.

The invention also pertains to regenerating or augmenting tissue and/ororgan function by recruiting native stem cells to a target site bydelivering a biocompatible substance to a target site. Biocompatiblerefers to materials that do not have toxic or injurious effects onbiological functions. Biodegradable refers to material that can beabsorbed or degraded in a patient's body. Representative materials forforming the biodegradable material include natural or syntheticpolymers, such as, collagen, poly(alpha esters) such as poly(lacticacid), poly(glycolic acid), polyorthoesters amd polyanhydrides and theircopolymers, which degraded by hydrolysis at a controlled rate and arereabsorbed. These materials provide the maximum control ofdegradability, manageability, size and configuration. Preferredbiodegradable polymer materials include polyglycolic acid andpolyglactin, developed as absorbable synthetic suture material.

Polyglycolic acid and polyglactin fibers may be used as supplied by themanufacturer. Other biodegradable materials include, but are not limitedto, cellulose ether, cellulose, cellulosic ester, fluorinatedpolyethylene, phenolic, poly-4-methylpentene, polyacrylonitrile,polyamide, polyamideimide, polyacrylate, polybenzoxazole, polycarbonate,polycyanoarylether, polyester, polyestercarbonate, polyether,polyetheretherketone, polyetherimide, polyetherketone, polyethersulfone,polyethylene, polyfluoroolefin, polyimide, polyolefin, polyoxadiazole,polyphenylene oxide, polyphenylene sulfide, polypropylene, polystyrene,polysulfide, polysulfone, polytetrafluoroethylene, polythioether,polytriazole, polyurethane, polyvinyl, polyvinylidene fluoride,regenerated cellulose, silicone, urea-formaldehyde, or copolymers orphysical blends of these materials. The material may be impregnated withsuitable antimicrobial agents and may be colored by a color additive toimprove visibility and to aid in surgical procedures.

The polymers can be characterized for mechanical properties such astensile strength using an Instron tester, for polymer molecular weightby gel permeation chromatography (GPC), glass transition temperature bydifferential scanning calorimetry (DSC) and bond structure by infrared(IR) spectroscopy; with respect to toxicology by initial screening testsinvolving Ames assays and in vitro teratogenicity assays, andimplantation studies in animals for immunogenicity, inflammation,release and degradation studies. In vitro cell attachment and viabilitycan be assessed using scanning electron microscopy, histology, andquantitative assessment with radioisotopes.

In some embodiments, the biocompatible substrate can be shaped usingmethods such as, solvent casting, compression molding, filament drawing,meshing, leaching, weaving and coating. In solvent casting, a solutionof one or more polymers in an appropriate solvent, such as methylenechloride, is cast as a branching pattern relief structure. After solventevaporation, a thin film is obtained. In compression molding, thesubstrate is pressed at pressures up to 30,000 pounds per square inchinto an appropriate pattern. Filament drawing involves drawing from themolten polymer and meshing involves forming a mesh by compressing fibersinto a felt-like material. In leaching, a solution containing twomaterials is spread into a shape close to the final form of the tissue.Next a solvent is used to dissolve away one of the components, resultingin pore formation. (See Mikos, U.S. Pat. No. 5,514,378, herebyincorporated by reference).

Thus, the substrate can be shaped into any number of desirableconfigurations to satisfy any number of overall system, geometry orspace restrictions. The biomaterial can be shaped to different sizes toconform to the necessary structures of different sized patients.

A substrate can also be permeated with a material, for example liquefiedcopolymers (poly-DL-lactide co-glycolide 50:50 80 mg/ml methylenechloride) to alter its mechanical properties. This can be performed bycoating one layer, or multiple layers until the desired mechanicalproperties are achieved.

In one embodiment, the biomaterial is an injectable biomaterial that canbe composed of crosslinked polymer networks which are typicallyinsoluble or poorly soluble in water, but can swell to an equilibriumsize in the presence of excess water. For example, the hydrogel can beinjected into desired locations within the organ. In one embodiment, thecollagen can be injected alone. In another embodiment, the collagen canbe injected with other hydrogels. The hydrogel compositions can include,without limitation, for example, poly(esters), poly(hydroxy acids),poly(lactones),poly(amides), poly(ester-amides), poly(amino acids),poly(anhydrides), poly(ortho-esters), poly(carbonates),poly(phosphazines), poly(thioesters), polysaccharides and ′ mixturesthereof. Furthermore, the compositions can also include, for example, apoly(hydroxy) acid including poly(alpha-hydroxy) acids andpoly(beta-hydroxy) acids. Such poly(hydroxy) acids include, for example,polylactic acid, polyglycolic acid, polycaproic acid, polybutyric acid,polyvaleric acid, and copolymers and mixtures thereof. Due to the uniqueproperties of hydrogels and their potential applications in such areasas controlled drug delivery, various types of hydrogels have beensynthesized and characterized.

The bulk polymerization, i.e., polymerization in the absence of addedsolvent, of monomers to make a homogeneous hydrogel produces a glassy,transparent polymer matrix which is very hard. When immersed in water,the glassy matrix swells to become soft and flexible. Porous hydrogelsare usually prepared by a solution polymerization technique, whichentails polymerizing monomers in a suitable solvent. The nature of asynthesized hydrogel, whether a compact gel or a loose polymer network,depends on the type of monomer, the amount of diluent in the monomermixture, and the amount of crosslinking agent. As the amount of diluent(usually water) in the monomer mixture increases, the pore size alsoincreases up to the micron range. Hydrogels with effective pore sizes inthe 10-100 nm range and in the 100 nm-10 micrometer range are termed“microporous” and “macroporous” hydrogels, respectively. The microporousand macroporous structures of hydrogels can be distinguished from thoseof non-hydrogel porous materials, such as porous polyurethane foams. Inthe plastic foam area, micro- and macro-pores are indicated as havingpores less than 50 micrometers and pores in the 100-300 micrometerrange, respectively. One of the reasons for this difference is thathydrogels with pores larger than 10 micrometers are uncommon, whileporous plastics having pores in the 100-300 micrometer range are verycommon.

Microporous and macroporous hydrogels are often called polymer“sponges.” When a monomer, e.g., hydroxyethyl methacrylate (HEMA), ispolymerized at an initial monomer concentration of 45 (w/w) % or higherin water, a hydrogel is produced with a porosity higher than thehomogeneous hydrogels. Hydrogels can also expand in the presence ofdiluent (usually water). The matrix materials of present inventionencompass both conventional foam or sponge materials and the so-called“hydrogel sponges.” For a further description of hydrogels, see U.S.Pat. No. 5,451,613 (issued to Smith et al).

Collagen gels can also be used. The collagen used in the presentinvention can be collagen such as Type I, Type III or Type I+IIIcollagen, for example, alkaline treatment of insoluble collagenextracted from various animals, or by treating with enzyme such aspepsin, trypsin, chymotrypsin, papain or pronase. There are noparticular restrictions on the origin of the collagen, and typicallycollagen can be used that is obtained from the skin, bone, cartilage,tendon or organs, etc. of birds or mammals such as cows, pigs, rabbits,sheep and mice. Since collagen allows the obtaining of a suitableconsistency without heating, preparation can be made easily in the caseof gelation. In addition, collagen has a high molecular weight, it moreclosely resembles living body tissue, has considerable physiologicalactivity, and therefore promotes healing in the case of using on awound, resulting in a favorable effect for tissue regeneration. Collagencan be flexible after curing and requires only a short time forcrosslinking, in other words, requires only a short time for gelation.Collagen solution can also be made by dissolving in a non-toxic solventwith respect to the living body, examples of which include water,physiological saline, a buffer such as borate buffer, or an aqueoussolution containing a salt such as sodium chloride, sodium bromide andpotassium bromide, or protein, sugar or lipid, etc.

The collagen can also form a gel even in the presence of moisture suchas that in blood or humor, and can demonstrate a high degree ofadhesiveness with respect to living body tissue. Collagen solutions usedin the present invention can be made at various concentrations,neutralized and prepared for injection. Preferably, collagen at 0.2mg/mL, 0.5 mg/ml, 0.75 mg/ml, 1 mg/ml, 2 mg/ml, 3 mg/ml, 4 mg/ml, 5mg/ml, 6 mg/ml, 7 mg/ml, 8 mg/ml, 10 mg/ml, 20 mg/ml, 30 mg/ml, 40 mg/mland 5 mg/ml in in solution can be used for injection. More preferably,collagen concentration from about 1 mg/ml to about 30 mg/ml solution canbe used for injection. Upon injection into an organ, chilled collagengels can thermogel as they reached body temperature or about 37° C.

The biocompatible substance can also have a range of viscosities. Insome aspects, the biocompatible substance has a viscosity between about5 cP to about 1×10⁸ cP, or about 5 cP to about 1×10⁶ cP, or about 5 cPto about 1×10⁵ cP, or about 5 cP to about 1×10⁴ cP, or about 10 cP toabout 1×10³ cP, or about 6 cP to about 9500 cP at 25° C.

III. Adjuvants

Substrates can be treated with additives or drugs prior to implantation(before or after the polymeric substrate is seeded with cells), e.g., toenhance native stem cell recruitment, and differentiation into newtissue after implantation. Thus, for example, growth factors, cytokines,extracellular matrix components, and other bioactive materials can beadded to the substrate to promote graft healing and formation of newtissue. Such additives will in general be selected according to thetissue or organ being reconstructed or augmented, to ensure thatappropriate new tissue is formed in the engrafted organ or tissue (forexamples of such additives for use in promoting bone healing, see, e.g.,Kirker-Head, C. A. Vet. Surg. 24 (5): 408-19 (1995)). For example,vascular endothelial growth factor (VEGF, see, e.g., U.S. Pat. No.5,654,273 herein incorporated by reference) can be employed to promotethe formation of new vascular tissue. Growth factors and other additives(e.g., epidermal growth factor (EGF), heparin-binding epidermal-likegrowth factor (HBGF), fibroblast growth factor (FGF), cytokines, genes,proteins, and the like) can be added in amounts in excess of any amountof such growth factors (if any) which may be produced by the cellsseeded on the substrate. Such additives are preferably provided in anamount sufficient to promote the formation of new tissue of a typeappropriate to the tissue or organ, which is to be repaired or augmented(e.g., by causing or accelerating infiltration of host cells into thegraft). Other useful additives include antibacterial agents such asantibiotics.

The substrate can also be treated or seeded with various factors andproteins to control the degradation/absorption of the biomaterial in thesubject. For instance, if the cells recruited to the biomaterial areslow-growing, then it is beneficial to maintain the biomaterialintegrity for a long enough period of time to allow the cells enoughtime to regenerate and grow. On the other hand, if the cells are able toquickly reproduce and grow, then a short lived substrate could bedesirable. Varying the concentration of aprotinin additives,aminocaproic acid, tranxemic acid, or similar fibrinolytic inhibitors orthe degree of chemical cross-linking in the biomaterial could be used toprecisely control this variable. The substrate could also be seeded withvarying growth factors.

A person skilled in the art will appreciate that the biocompatiblesubstances can have a variety of other configurations and can includevarious other features known in the art. In some embodiments, thebiocompatible substance of the present invention can also be used withsuitable adjuvants for improving the target site for in situ tissueregeneration. As such the invention extends to compositions aspreviously defined, additionally comprising one or more adjuvants. Forexample, non-limiting suitable adjuvants include the general classes of:antibacterial agents, such as metronidazole, silver;anaesthetic/analgesics, such as lidocaine, benzovaine; anti-inflammatoryagents, such as steroidal, non-steroidal; collagen synthetaseinhibitors, growth factors, such as transforming growth factor beta,endothelial growth factor, basic fibroblast growth factor, nerve growthfactor; enzymes for debridement, such as subtilysin, bromain, papain;and genes for gene therapy, such as the vascular endothelial GF-2 (VEGF)angiogenesis gene. In some embodiments, one or more anti-inflammatoryagents can be delivered to the target site, and/or incorporated into thebiocompatible substance. In preferred embodiments, the anti-inflammatoryagent is a collagen reducing agent. Anti-inflammatory agents can besteroidal or non-steroidal. Non-limiting examples of anti-inflammatoryagents include; corticosteroids, dexamethasone, collagenase, rapamycin,paclitaxel, ABT-578, everolimus, taxol, steroidal anti-inflammatoryagents, non-steroidal anti-inflammatory agents, hemostatic agents;antimicrobial agents; antibiotics; antifungals; antiprotozoals;antivirals; antimicrobial metals; hemostatic and/or vasoconstrictingagents; pseudoephedrine; xylometazoline; oxymetazoline; phenylephrine;epinephrine; cocaine; local anesthetic agents; lidocaine; cocaine;bupivacaine; hormones; hormonally active agents; agents that enhancepotency; substances that dissolve, degrade, cut, break, weaken, soften,modify or remodel connective tissue or other tissue; enzymes; trypsin;EDTA; trypsin combined with EDTA; hyaluronidase; tosyllysylchloromethane(TLCM)); chemotherapeutic or antineoplastic agents; substances thatprevent adhesion formation; hyaluronic acid gel. Non-limiting examplesof collagen synthetase inhibitors include collagenase, halofuginone,propyl hydroxylase, c-proteinase inhibitor, and metalloproteinase. Itwill be appreciated that other suitable compounds can be used, asappropriate.

Adjuvants can be any suitable therapeutic or biological agent such asgenetic material, growth factors, cytokines, enzymes. The adjuvant canalso be released at a specific site as a function of biodegradation ofthe biomaterial in the surrounding environment over time.

Examples of adjuvants include, but are not limited to proteins growthfactors, antibodies, nucleic acids molecules, carbohydrates,anti-inflammatory agents, and the like. Growth factors useful in thepresent invention include, but are not limited to, transforming growthfactor-alpha (TGF-α), transforming growth factor-beta (TGF-β),platelet-derived growth factors (PDGF), fibroblast growth factors (FGF),including FGF acidic isoforms 1 and 2, FGF basic form 2 and FGF 4, 8, 9and 10, nerve growth factors (NGF) including NGF 2.5 s, NGF 7.0s andbeta NGF and neurotrophins, brain derived neurotrophic factor, cartilagederived factor, bone growth factors (BGF), basic fibroblast growthfactor, insulin-like growth factor (IGF), vascular endothelial growthfactor (VEGF), granulocyte colony stimulating factor (G-CSF), insulinlike growth factor (IGF) I and II, hepatocyte growth factor, glialneurotrophic growth factor (GDNF), stem cell factor (SCF), keratinocytegrowth factor (KGF), transforming growth factors (TGF), including TGFsalpha, beta, beta1, beta2, beta3, skeletal growth factor, bone matrixderived growth factors, and bone derived growth factors and mixturesthereof.

Cytokines useful in the present invention include, but are not limitedto, cardiotrophin, stromal cell derived factor, macrophage derivedchemokine (MDC), melanoma growth stimulatory activity (MGSA), macrophageinflammatory proteins 1alpha (MIP-1 alpha), 2, 3 alpha, 3 beta, 4 and 5,IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-11,IL-12, IL-13, TNF-alpha, and TNF-beta. Immunoglobulins useful in thepresent invention include, but are not limited to, IgG, IgA, IgM, IgD,IgE, and mixtures thereof. Some preferred growth factors include VEGF(vascular endothelial growth factor), NGFs (nerve growth factors),PDGF-AA, PDGF-BB, PDGF-AB, FGFb, FGFa, and BGF.

In some embodiments, one or more anti-inflammatory agents can bedelivered to the target site, and/or incorporated into the biocompatiblesubstance. In preferred embodiments, the anti-inflammatory agent is acollagen reducing agent. Anti-inflammatory agents can be steroidal ornon-steroidal. Non-limiting examples of anti-inflammatory agentsinclude, corticosteroids, dexamethasone, collagenase, rapamycin,paclitaxel, ABT-578, everolimus, taxol, steroidal anti-inflammatoryagents, non-steroidal anti-inflammatory agents, hemostatic agents;antimicrobial agents; antibiotics; antifungals; antiprotozoals;antivirals; antimicrobial metals; hemostatic and/or vasoconstrictingagents; pseudoephedrine; xylometazoline; oxymetazoline; phenylephrine;epinephrine; cocaine; local anesthetic agents; lidocaine; cocaine;bupivacaine; hormones; hormonally active agents; agents that enhancepotency; substances that dissolve, degrade, cut, break, weaken, soften,modify or remodel connective tissue or other tissue; enzymes; trypsin;EDTA; trypsin combined with EDTA; hyaluronidase; tosyllysylchloromethane(TLCM)); chemotherapeutic or antineoplastic agents; substances thatprevent adhesion formation; hyaluronic acid gel. Non-limiting examplesof collagen synthetase inhibitors include collagenase, halofuginone,propyl hydroxylase, c-proteinase inhibitor, and metalloproteinase. Itwill be appreciated that other suitable compounds may be used, asappropriate.

Other molecules useful as therapeutic or biological agents include, butare not limited to, growth hormones, leptin, leukemia inhibitory factor(LIF), endostatin, thrombospondin, osteogenic protein-1, bonemorphogenetic proteins 2 and 7, osteonectin, somatomedin-like peptide,osteocalcin, interferon alpha, interferon alpha A, interferon beta,interferon gamma, interferon 1 alpha.

Embodiments involving amino acids, peptides, polypeptides, and proteinsmay include any type or combinations of such molecules of any size andcomplexity. Examples include, but are not limited to structuralproteins, enzymes, and peptide hormones. These compounds can serve avariety of functions. In some embodiments, the biomaterial can containpeptides containing a sequence that suppresses enzyme activity throughcompetition for the active site. In substances such as nucleic acids,any nucleic acid can be present. Examples include, but are not limitedto deoxyribonucleic acid (DNA), and ribonucleic acid (RNA). Embodimentsinvolving DNA include, but are not limited to, cDNA sequences, naturalDNA sequences from any source, and sense or anti-sense oligonucleotides.For example, DNA can be naked (e.g., U.S. Pat. Nos. 5,580,859;5,910,488) or complexed or encapsulated (e.g., U.S. Pat. Nos. 5,908,777;5,787,567). DNA can be present in vectors of any kind, for example in aviral or plasmid vector. In some embodiments, nucleic acids used willserve to promote or to inhibit the expression of genes in cellsrecruited inside to the biomaterial. The nucleic acids can be in anyform that is effective to enhance its uptake into cells.

The state of the biomaterial in relation to the adjuvant can becontrolled by the coupling chemistry, whether the therapeutic/biologicalagent is encapsulated, the selection of biomaterial compounds,solvent(s), and solubility of the biomaterial compounds in thosesolvents. These parameters can be manipulated to control the release ofthe therapeutic/biological agents. It is to be understood that theadjuvant can be entrapped or entangled within a biomaterial, bonded to abiomaterial, contained within cavities, enclosures, inclusions, orpockets, or structures of a biomaterial (e.g. fibers, fibrils,particles) or externally bound to the biomaterial.

In particular, the adjuvant can be entrapped or encapsulated to produce“nanocapsules.” These nanocapsules containing the adjuvant can beproduced by standard encapsulating techniques. Microencapsulation ofadjuvants generally involve three steps: (a) generating microcapsulesenclosing the adjuvant (e.g., by forming droplets of cell-containingliquid alginate followed by exposure to a solution of calcium chlorideto form a solid gel), (b) coating the resulting gelled spheres withadditional outer coatings (e.g., outer coatings comprising polylysineand/or polyornithine) to form a semipermeable membrane; and (c)liquefying the original core gel (e.g., by chelation using a solution ofsodium citrate). The three steps are typically separated by washings innormal saline.

Another method of microencapsulating adjuvants can be thealginate-polyamino acid technique. Droplets of sodium alginate areproduced. Droplets of alginate flow into calcium chloride in saline. Thenegatively charged alginate droplets bind calcium and form a calciumalginate gel. The microcapsules are washed in saline and incubated withpoly-L-lysine (PLL) or poly-L-ornithine (or combinations thereof); thepositively charged poly-l-lysine and/or poly-L-ornithine displacescalcium ions and binds (ionic) negatively charged alginate, producing anouter poly-electrolyte membrane. A final coating of sodium alginate maybe added by washing the microcapsules with a solution of sodiumalginate, which ionically bonds to the poly-L-lysine and/orpoly-L-ornithine layer. See U.S. Pat. No. 4,391,909 to Lim et al (allU.S. patents referenced herein are intended to be incorporated herein intheir entirety). This technique produces what has been termed a“single-wall” microcapsule. Preferred microcapsules are essentiallyround, small, and uniform in size. (Wolters et al., J. Appli Biomater.3:281 (1992)).

The alginate-polylysine microcapsules can also then be incubated insodium citrate to solubilize any calcium alginate that has not reactedwith poly-1-lysine, i.e., to solubilize the internal core of sodiumalginate, thus producing a microcapsule with a liquefied core portion.See Lim and Sun, Science 210:908 (1980). Such microcapsules are referredto herein as having “chelated”, “hollow” or “liquid” cores. A“double-wall” microcapsule is produced by following the same procedureas for single-wall microcapsules, but prior to any incubation withsodium citrate, the microcapsules are again incubated with poly-l-lysineand sodium alginate.

Many alternative techniques used for encapsulating different agents areknown in the art and can be used with this invention. U.S. Pat. No.5,084,350 discloses microcapsules enclosed in a larger matrix, where themicrocapsules are liquefied once the microcapsules are within the largermatrix. Tsang et al., U.S. Pat. No. 4,663,286 discloses encapsulationusing an alginate polymer, where the gel layer is cross-linked with apolycationic polymer such as polylysine, and a second layer formed usinga second polycationic polymer (such as polyornithine); the second layercan then be coated by alginate. U.S. Pat. No. 5,762,959 to Soon-Shionget al. discloses a microcapsule having a solid (non-chelated) alginategel core of a defined ratio of calcium/barium alginates, with polymermaterial in the core. U.S. Pat. Nos. 5,801,033 and 5,573,934 to Hubbellet al. describe alginate/polylysine microspheres having a finalpolymeric coating (e.g., polyethylene glycol (PEG)); Sawhney et al.,Biomaterials 13:863 (1991) describe alginate/polylysine microcapsulesincorporating a graft copolymer of poly-1-lysine and polyethylene oxideon the microcapsule surface, to improve biocompatibility; U.S. Pat. No.5,380,536 describes microcapsules with an outermost layer of watersoluble non-ionic polymers such as polyethylene(oxide). U.S. Pat. No.5,227,298 to Weber et al. describes a method for providing a secondalginate gel coating to cells already coated with polylysine alginate;both alginate coatings are stabilized with polylysine. U.S. Pat. No.5,578,314 to Weber et al. provides a method for microencapsulation usingmultiple coatings of purified alginate. U.S. Pat. No. 5,693,514 toDorian et al. reports the use of a non-fibrogenic alginate, where theouter surface of the alginate coating is reacted with alkaline earthmetal cations comprising calcium ions and/or magnesium ions, to form analkaline earth metal alginate coating. The outer surface of the alginatecoating is not reacted with polylysine. U.S. Pat. No. 5,846,530 toSoon-Shiong describes microcapsules containing cells that have beenindividually coated with polymerizable alginate, or polymerizablepolycations such as polylysine, prior to encapsulation,

An adjuvant can be coupled to a nanoparticle and release kinetics of theadjuvant can be controlled. One skilled in the art will appreciate thatthe control release kinetics depend on the capsulation parametersincluding nanocapsule size, adjuvant loading, and polymer composition.The mean diameter of the nanocapsules depends on the mixing velocity ofthe preparation process and viscosity of the preparation media.Nanocapsule size can be reduced by exposing the preparation tosonication over a range of about 30 second to about 120 seconds,increasing the sonication intensity from about 5 watts to about 20watts, or by varying the ratios of organic polymer phase to aqueousphase. Nanocapsule sizes can be characterized by scanning electronmicroscopy (SEM), coulter counter, and light scattering.

For polymer encapsulation, FDA approved biodegradable polymers (PLA,PLGA, PCL) can be used for the control of encapsulation and degradationof the nanocapsules in vivo.

In one embodiment, the adjuvant can be joined to the biomaterial bypeptide bonds. For example, nanoparticles can be incorporated into thebiomaterial using EDC (1-ethyl-3(3-dimethly aminopropyl) carbodiimide)and sulfo-NHS (N-hydrocyl-sulfo-succinimide) to form peptide bonds.Various other know techniques can be used as described, for example, inHeumanson, Bioconjugate Techniques, Academic Press San Diego, Calif.,1996, herein incorporated by reference. For external incorporation, apeptide bond can be created between the biomaterial and the adjuvantusing the EDC/sulpho-NHS method to form peptide bonds between thecarboxylates and amino groups. The adjuvant can also be added internallyto the biomaterial by incorporating each component into the solutionwith at least one natural compound and at least one synthetic compound.

Examples of some possible chemistries of incorporating agents include,but are not limited to, esterification (e.g., with acyl halidcs, acidanhydrides, carboxylic acids, or esters via interchange reactions),ether formation (for example, via the Williamson ether synthesis),urethane formation via reactions with isocyanates, sulfonation with, forexample, chlorosulfonic acid, and reaction of b-sulfato-ethylsulfonylaniline to afford an amine derivative that can be converted to a diazofor reaction with a wide variety of compounds. Such chemistries can beused to attach a wide variety of substances to the biomaterial,including but not limited to crown ethers (Kimura et al., (1983) J.Polym. Sci. 21, 2777), enzymes (Chase et al. (1998) Biotechnol. Appl.Biochem., 27, 205), and nucleotides (Overberger et al. (1989) J. Polym.Sci. 27, 3589).

IV. Injection and Implantation of Biomaterials

The substantially cell-free biomaterial can be implanted or injectedinto an organ requiring regeneration or organ augmentation usingstandard surgical procedures. These surgical procedures may varyaccording to the organ being augmented. For kidney implantation, it maybe desirable to implant a series of substantially cell-free biomaterialsinto incisions formed along the avascular plane of the kidney, or theleast vascular region of an organ. In other applications, the constructsof the invention can be introduced by less invasive procedures, e.g.,via a cannula, needle, trocar or catheter-type instrument.

A person skilled in the art will appreciate that the biocompatiblesubstance can be delivered/implanted by a variety of methods known inthe art. Other embodiments and uses of the invention will be apparent tothose skilled in the art from consideration of the specification andpractice of the invention disclosed herein. All U.S. patents and otherreferences noted herein for whatever reason are specificallyincorporated by reference. The specification and examples should beconsidered exemplary only with the true scope and spirit of theinvention indicated by the claims.

EXAMPLES

The examples demonstrate that host cell infiltrates into a biomaterialimplant are not entirely comprised of inflammatory and fibroblast-likecells, and that normal inflammatory process can be altered byincorporating agents that influence environmental cues. The infiltratedcells are capable of differentiating into multiple cell lineages ifappropriate conditions are provided. It is possible to recruit apredominance of native cells with multilineage potential into abiomaterial scaffold. Therefore, it is possible to enrich the infiltratewith such cell types and control their fate, provided the propersubstrate-mediated signaling can be imparted into the scaffold.

Example 1: Methods and Materials Normal Mouse Model

All animal procedures were performed in accordance with a protocolapproved by the institutional Animal Care and Use Committee (ACUC) atWake Forest University. CD1 mice (6-8 weeks) were purchased from CharlesRiver Laboratories Inc. (Wilmington, Mass.). Under anesthesia usingisoflurane, the kidneys were accessed through a dorsal incision and thencollagen gels were injected into kidneys. Mice were divided into threeexperimental groups (n=20 per time point). The following treatments wereadministered via multiple injections into both kidneys using a 22-gaugeneedle: (1) collagen gel (0.2% wt/vol, 50 nL/kidney), (2) saline (0.9%NaCl, 50 uL/kidney, Hospira, Inc, Lake Forest, Ill.), and (3) needlesticks only. Sham operation served as a control. The kidneys wereharvested at 1, 2, 3, and 4 weeks after injection for histological andimmunohistochemical analyses

Collagen Gel Preparation

Rat tail type I collagen solution was obtained from BD Biosciences(Franklin Lakes, N.J.). Collagen gels were prepared on ice. Briefly,collagen solution (0.2% wt/vol) was neutralized by adding IN NaOHsolution to give a final pH of 7.4. Neutralized collagen gels weretested to achieve optimal injection conditions prior to injection. Uponinjection into the kidney, the collagen gels thermogelled as theyreached 37° C. All chemicals were obtained from Sigma-Aldrich Co. (StLouis, Mo.) and used as received unless stated otherwise.

Renal Ischemia/Reperfusion Rat Model

Seventy Lewis rats (6-8 weeks, weight approximately 200 g, Charles RiverLaboratories Inc.) were anesthetized with an intraperitoneal injectionof sodium pentobarbital (Nembutal, Ovation Pharmaceuticals, Inc.,Deerfield, Ill.) at an initial dose of 50 mg/kg. If necessary,anesthesia was maintained using a second 25 mg/kg dose. The kidneys wereexposed through a 3 cm midline abdominal incision. The bilateral renalpedicles were isolated. Each renal artery and vein was occluded withnon-traumatic clamps (Micro-serrefine curved 6 mm, Fine Science ToolsInc. Foster, Calif.) for 60 min. At the end of this time period, theclamps were released to allow renal reperfusion. The abdominal wall wasclosed in two layers. Post surgical pain was managed with buprenorphine(Reckitt Benckiser Pharmaceuticals, Richmond, Va., 0.05 mg/Kgsubcutaneously). Three weeks after ischemia/reperfusion surgery, aneutralized collagen gel (0.2% wt/vol, 400 uL) was injected intomultiple areas of the kidneys using a 20-gauge needle. Sham operationand saline injection groups served as controls as described above.

Scaffold Implantation

Nonwoven poly(glycolic acid) (PGA; density 50 mg/cc, thickness 2 mm) wasused as a polymeric scaffold to accommodate host cell infiltration andwas obtained from Biomedical Structures, Inc. (Slaterville, R.I., USA).All chemicals were obtained from Sigma-Aldrich Co. (St Louis, Mo., USA)and used as received unless stated otherwise.

In a separate set of experiments, dexamethasone, which is a knownanti-inflammatory agent, was incorporated into the PGA scaffolds todetermine whether normal host tissue response could be altered. PGAscaffolds (8*8×2 mm) were treated with dexamethasone (0.2 mg/mL)-loadedPluronic F127 hydogel. The scaffolds 1 were implanted subcutaneouslyunder the dorsal skin of CD1 mice, and retrieved at 3 and 4 weeks afterimplantation. PGA scaffolds with Pluronic F127 hydrogel only served ascontrols. The retrieved scaffolds were assessed for DNA and solublecollagen content.

Scanning Electron Microscope (SEM)

Morphology of the implanted PGA scaffolds was examined by SEM (ModelS-2260N, Hitachi Co. Ltd., Tokyo, Japan). Samples were observed under anenvironmental SEM (backscatter electron mode) without any conductivecoating. To observe morphologies, samples were fixed with 1%glutaraldehyde solution and dehydrated through a series of gradedethanol solutions, followed by observation with SEM.

Measurement of Cellular Component in Implants

Retrieved scaffolds were fixed in 10% buffered formalin, sectioned, andstained with hematoxylin and eosin (H&E). The number of infiltratingcells was counted in representative sections. Cell numbers were measuredin each section using nine randomly selected neighboring fields of equalarea and averaged. Counts were expressed as average number per mm².

Retrieved scaffolds were also analyzed for DNA content. The DNA waspurified using DNeasy® kit (QIAGEN Inc., Valencia, Calif., USA) andconcentration was measured by spectrophotometric analysis (BioMate 3,Thermo Electron Corporation, Waltham, Mass., USA).

Measurement of Collagen Content

Collagen content was assessed with Masson's trichrome staining.Retrieved scaffolds were fixed in 10% buffered formalin and Bouin'sfixation solution and sectioned. The slides were placed in Weigert'siron hematoxylin working solution followed by rinse with water.Subsequently, the slides were placed in Biebrich Scarlet-acid fuchsin,incubated with phosphotungstic-phosphomolybdic acid and stained withaniline blue. Photomicrographs were taken using a Nikon lightmicroscope.

The retrieved scaffolds were quantitatively analyzed for total collagencontent. Total soluble collagen was extracted using 1 mg/ml pepsin in0.5 M acetic acid for 72 hr at 4° C. The samples were centrifuged andsupernatants stored for further analysis. The collagen concentration wasmeasured using a Sircol™ Soluble Collagen Assay kit (Biocolor Ltd.,Belfast, Northern Ireland) according to the manufacturer's instructions.The dye in the kit binds specifically to the sequence [Gly-X-Y]_(n)present in all collagen types. Collagen content was normalized to theimplanted scaffold wet weight.

Culture of Infiltrating Cells

The implanted PGA scaffolds were retrieved at their predetermined timepoints. The infiltrating cells were isolated by cutting the scaffoldsinto small pieces (1 mm³), and digesting in sterile phosphate-bufferedsaline containing 1.25 mg/ml collagenase type I (Worthington BiochemicalCorporation, Lakewood, N.J., USA) for 2 hr at 37° C. The cells wereresuspended in culture medium, plated on tissue culture dishes and grownto confluence for 2-3 weeks at 5% CO₂, 95% humidity and 37° C. Unlessindicated otherwise, original culture medium consisted of low-glucoseDulbecco's modified Eagle medium (DMEM) supplemented with 10% fetalbovine serum (FBS) and 1% penicillin/streptomycin. The cells weresubcultured using 0.25% trypsin containing 1 raM EDTA for 5 min at 37°C. All cells were cultured at 5% CO₂, 37° C., and 95% humidity unlessotherwise indicated.

FACS Analysis and IF Staining

Fluorescence-activated cell scanning (FACS) analysis was performed forCD34, CD45, CD90, Sca-1, and Flk-1. For all antibodies, 0.5×10⁶ of theinfiltrating cells were incubated in 100 uL of PBS containing 1% FBS andprimary antibody at dilutions ranging from 1:15 to 1:100. Cells wereincubated with primary antibody on ice for 30 min, washed with 1% FBS inPBS, resuspended in 100 ul of fluorescein isothiocyanate (FITC)-labeledsecondary antibody, diluted 1:100 in 1% FBS in PBS, and incubated for anadditional 30 min on ice. The cells were then washed with PBS containing1% FBS, and resuspended in PBS with 1% FBS for FACS analysis.Isotype-matching immunoglobulin (IgG) and FITC-labeled secondaryantibody were used to determine nonspecific signals. FACS analyses wereperformed with a FACSCalibur flow cytometer (BD FACSCalibur System, SanJose, Calif., USA) equipped with an air cooled argon laser (588-nmemission). The infiltrating cells were also characterized byimmunofluorescent (IF) staining using anti-CD34 and rat anti-mouse Sca-1(1:200, BD Biosciences Pharmingen, San Diego, Calif.) as the primaryantibodies. Samples were incubated with primary antibody for 1 hr atroom temperature with subsequent washing in PBS followed by incubationwith the secondary antibody (FITC-conjugated horse antibody to mouseIgG, 1:500, DakoCytomation, Carpinteria, Calif., USA) for 30 min. AfterIF staining, the cells were viewed using a fluorescence microscope(Nikon, Japan). Samples stained without primary antibody served as anegative control.

Multilineage Differentiation: Oxteosenic Induction

To determine whether infiltrating cells could undergo osteogenicdifferentiation, cells at passage four were plated at a density of 5,000cells/cm² and cultured in low-glucose DMEM medium with 10% FBS, 1%penicillin/streptomycin, osteogenic supplements (100 nM dexamethasone,10 mM β-glycerophosphate and 0.05 mM ascorbic acid-2-phosphate). As acontrol, cells were also cultured in the original culture medium. Thecells were cultured in osteogenic medium for up to 30 days with mediachanges every 3 days.

Cellular mineralization was determined by von Kossa staining. The cellswere fixed with 10% formaldehyde, incubated with 2% silver nitratesolution for 10 min, washed with deionized water, and exposed to UVlight for 15 min.

Endothelial Induction

For the induction of endothelial differentiation, passage four cellswere plated at a density of 5,000 cells/cm² and subsequently cultured inEGM-2 (Endothelial growth medium-2, CAMBREX, Walkersville, Md., USA)culture media. The cells were sub-cultured on 6-well tissue cultureplates coated with Matrigel™ (BD Biosciences, Bedford, Mass., USA) toassess capillary formation. After allowing 30 minutes after initialseeding for cell attachment, EGM-2 was added. Twelve hours later, thecells were examined under a phase-contrast microscope (Nikon, Japan) forevidence of capillary formation.

Adipogenic Induction

For induction of adipogenic differentiation, passage four cells wereplated at a density of 5,000 cells/cm², allowed to adhere, and culturedafter submersion in low-glucose DMEM supplemented with 10% FBS, 1%penicillin/streptomycin, and adipogenic supplements (1 μM dexamethasone,1 mM 3-isobutyl-1-methylxanthine, 10 μg/ml insulin and 60 μMindomethacin). The cells cultured in the original culture medium servedas a control. The medium was changed every 3 days.

The presence of intracellular lipid vacuoles was determined withOil-Red-O. The cells were incubated with Oil-Red-O staining solution,rinsed with 50% ethanol, 2rinsed again with distilled water,counterstained with Gill's hematoxylin, rinsed in deionized water andmounted with water-based mounting media.

Myogenic Induction

For the induction of myogenic differentiation, passage four cells wereplated at a density of 5,000 cells/cm² on dishes and cultured withmyogenic media (low-glucose DMEM supplemented with 10% horse serum, 0.5%chick embryo extract and 1% penicillin/streptomycin). After a 12-hrequilibration period, 5-azacytidine (10 μM) was added for 24 hr and thenthe media was replaced with 5-azacytidine-free medium. As a control,some cells were cultured in the original culture medium. Culture mediumwas changed every 3 days.

Immunohistochemical Analyses

The differentiated cells were detected by anti-osteocalcin antibodies(Santa Cruz Biotechnology, Inc., Santa Cruz, Calif., USA) for osteogenicinduction, anti-PECAM-1 and anti-von Willebrand factor (vWF) forendothelial induction, and anti-a-smooth muscle actin and anti-desminfor myogenic induction.

Briefly, the differentiated cells in 6-well plates were fixed with coldethanol (−20° C.). Nonspecific protein binding was blocked with 10%serum in PBS for 1 hr at room temperature, and cells subsequentlyincubated with primary antibody overnight at 4° C. The cells were washedthoroughly with PBS and incubated with FITC-conjugated secondaryantibody for 1 hr at room temperature. The cell were washed with PBS andmounted with a solution containing DAPI to detect nuclei (VectaShield,Vector Labs, Burlingame, Calif., USA).

Identification of Host Cells

In order to examine the infiltrating host cells into the injectionregions, immunohistochemical staining for bromodeoxyuridine (BrdU) andproliferating cell nuclear antigen (PCNA, Dako) was performed asdescribed above. For BrdU labeling, BrdU (100 mg/kg) was injectedintraperitoneally into normal CD1 mice daily for 2 weeks.

Quantitative analysis of PCNA-positive cells was performed by countingthe 2positive nuclei in injection and normal regions from five randomlyselected fields under a light microscope at magnification ×200.

Characterization of Newly Formed Renal Tissues

The regenerated renal structures were identified by immunohistochemistrywith cell specific markers: mouse anti-human CD31 (Dako), mouse anti-ratsynaptopodin (Fitzgerald industries International, Concord, Mass.),cytokeratin (Dako), and polyclonal rabbit anti-neprilysin (1:100,Millipore, Billerica, Mass.). To quantify the effects of the injectionof collagen gel, the number of glomeruli in the injection regions andnormal regions was counted and quantified.

Functional Testing

Blood samples for creatinine and blood urea nitrogen (BUN) determinationwere collected. Briefly, blood samples were collected from rat tailartery at weekly intervals from 1 week before ischemia/reperfusioninjury until 3 weeks after surgery for all the rats. Serum creatinineand BUN were measured using an automatic modular analyzer (Synchron CX5delta, Beckman Coulter Inc., Brea, Calif.).

Positive Pressure Measurement During the Injection

Animals (mice and rats) were anesthetized using 3% isoflurane in 100%oxygen via nasal cannel. The 22 gauge needle and the sensor frompressure transducer (blunt 26 gauge needle, pressure transducer fromADInstrument Inc. Colorado Spring, Colo.) were fixed together and thetip of the needle and the sensor was at the same level. Afterlaparotomy, saline (0.9% NaCl, 0.4 ml, Abbot Inc.) and collagen (1, 2,and 5 mg/ml, 0.4 ml per injection, BD Biosciences, Bedford, Mass.) wasinjected into the low pole renal parenchyma through 22 gauge needle (N=3kidneys per concentration). The pressure during injection was recordedvia data acquisition system (PowerLab 8/30, ADInstrument Inc. ColoradoSpring, Colo.).

Space

Mice were anesthetized using 3% isoflurane in 100% oxygen via nasalcannel. The kidney was approached through laparotomy. A space-specificlesion was made using 2 mm biopsy punch (Miltrex Inc., York, Pa.) at thelow pole of each kidney. The hole was about 2 mm depth. After removedthe renal tissue from the hole, hemostasis was obtained via directcompression with sterile cotton sticks. Collagen gel (2 mg/ml, BDBiosciences, Bedford, Mass.) was placed to the right hole and made thehole full. The hole on left kidney was left blank. The wound was closedin two layers. Mice at 2 weeks and 4 weeks after surgery were sacrificedand the kidneys were harvest for further evaluation.

Multiple Injections

After ischemia reperfusion surgery, collagen gels were injected into thekidney. The collagen gels were injected in the same fashion as describedabove. The additional injections were performed twice at the interval of2 weeks from previous injection. All of the rats were sacrificed at 2weeks after third injection.

Statistical Analysis

Data from the DNA content, cell number, and collagen content assays wereanalyzed by a single-factor analysis of variance (ANOVA). Differenceswere considered significant p<0.05.

Example 2: Scaffold Implantation

The PGA nonwoven scaffolds consisted of highly porous fiber mesh disks.The 1 retrieved scaffolds at 1, 2, 3 and 4 weeks showed a progressivetissue ingrowth over time. Cell recruitment after implantation wasmeasured by gross morphology and SEM microphotograph. By the fourthweek, the implants were completely encapsulated by a connective tissuecapsule with abundant host vasculature. Neovascularization was observedin and around the scaffold. Examination of the retrieved implants withscanning electron microscopy (SEM) clearly demonstrated a classicforeign body reaction. The porous scaffold became populated withcollagen-producing cells and the interfiber spaces were filled withincreasing amounts of collagenous matrix when measured by SEMmicrophotographs each week after implantation for 4 weeks. Observationsafter implantation, noted extracellular matrix material gradually filledin 2 the porous architecture of the scaffold.

Example 3: Cellular Component Measurement

Measurement of cell density within the scaffold was achieved byhistomorphometry and DNA content analyses (FIG. 1). DNA analysis,performed using a standard extraction kit, confirmed the typical hostcellular response. Cell infiltration increased steadily in the firstthree weeks, followed by a slight decline as matrix production increasedby the fourth week. These results were confirmed by histomorphometry,where the average cell counts of representative H&E stained sectionsshowed a similar pattern.

Example 4: Collagen Content Measurement

Histological analysis of collagen deposition within the scaffoldsprovided confirmation of the initial SEM images of retrieved implants.Masson's trichrome staining of representative sections after 1, 2, 3,and 4 weeks of implantation showed the gradual buildup of extracellularmatrix. The average collagen content of representative tissue samplesshowed a gradual increase in collagen production by the host cellinfiltrate over time (FIG. 2).

Example 5: Characterization of Infiltrating Cells

The host cell infiltrate was isolated by standard explant cultureisolation methods land the growth of these cells was observed overseveral passages. The cells grew well in culture and were subculturedwith relative ease. Microscopic observations of the cultured cellsshowed the presence of at least four different cell phenotypes of theinfiltrating cells. Flow cytometry was performed for severalhematopoietic and mesenchymal stem cell markers, as well as endothelialprogenitor cell markers (Sca-1 (FIG. 3A), Flk-1 (FIG. 3B), CD44 (FIG.3C), CD45 (FIG. 3D), CD31 (FIG. 3E), CD34 (FIG. 3F), CD90 (FIG. 3G) andCD117 (FIG. 3H)). Immunocytochemical staining of cultures ofinfiltrating cells showed a strong and persistent expression of Sca-1 atall time points and at all passages. However, the expression ofmesenchymal stem cell and endothelial progenitor cell markers wereabsent from these cell populations.

Example 6: Differentiation of Infiltrating Cells (i) OsteogenicDifferentiation

Recruiting cells were grown in osteogenic-inducing or original culturemedium and analyzed using Alizarin Red staining after 4, 8, 16, 24 and32 days in culture. The cells cultured in osteogenic medium up to 32days showed an intense Alizarin red staining which indicates calciumdeposition. The osteogenic induced cells showed significantmineralization after 16 days. The cells grown in the original culturemedium, however, fail to stain for calcium deposition. Quantitativeanalysis of calcium deposition showed a significant increase after 16days. The cells demonstrated an approximately thirteen-fold increase incalcium deposition in the osteogenesis-inducing medium compared to cellsgrown in original culture medium at day 32 (FIG. 4). Von Kossa stainingof cells grown in the osteogenic media demonstrated enhanced silvernitrate precipitation by day 16, indicating a high calcium level. Thecells grown in original culture medium showed no silver nitrateprecipitation over the 16-day period.

Immunocytochemical analysis with osteocalcin antibodies showed astrongly positive expression after 32 days of osteogenic induction.

(ii) Endothelial Differentiation

Infiltrated cells cultured in endothelial EGM showed the typicalendothelial differentiation of recruiting cells, “cobblestone”morphology of endothelial cells, after being grown inendothelial-inducing culture medium for 14 days. Immunocytochemicalassessment showed strong positive staining for PECAM-1 and vonWillebrand factor (vWF). Moreover, the cells formed capillary-likenetwork structures when cultured on Matrigel™. There was no obviousdifference observed in the capillary formation regardless of the passagetested. The cells grown in original culture medium showed nophenotypicexpression over the 16-day period.

(iii) Adipogenic Differentiation

In vitro incubation of the infiltrating cells in adipogenic media for 16days induced changes in cellular morphology. The cells lost theiroriginal elongated shape and became rounded. After 30 days in culture,the cytoplasm had completely filled with vacuoles that stainedpositively with Oil-Red-O, a common stain for lipid accumulation, whichindicates an adipose-like phenotype. The cells grown in original culturemedium showed no lipid accumulation over the 30-day period.

(iv) Myogenic Differentiation

Treatment of the cells with 5-azacytidine for 24 hr followed byincubation in myogenic media induced fusion of the cells to yieldmultinucleated clusters detected by phase contrast microscopy. The fusedcells formed myofiber-like structures after 16 days in myogenic medium.Immunocytochemistry showed strong positive expression for a-smoothmuscle actin and desmin after 16 days in culture. The cells grown inoriginal culture medium showed no myogenic expression over the 16-dayperiod.

Example 7: Dexamethasone-Incorporated PGA Scaffolds

Measurement of cellular components within the dexamethasone-incorporatedPGA scaffolds was accomplished by DNA content measurement. Cellularitywithin the control scaffolds were slightly declined at 4 weeks afterimplantation. However dexamethasone-incorporated PGA scaffolds showed agradual increase in DNA content by the same time point (FIG. 5A).Interestingly, collagen content within the dexamethasone-incorporatedPGA scaffolds was significantly reduced as compared to the controlscaffolds (P<0.05) (FIG. 5B).

Example 8: Regeneration Potential of Renal Tissue Using InjectableHydrogels

Various gel scaffolds including collagen based gel scaffolds, and otherbiomaterials, including collagen type I, collagen based kidney tissuegel matrix, synthetic gel matrix and keratin based gel matrix weretested in regeneration of kidney tissues. The results of all injectionsdemonstrated similar findings with formation of glomerular and tubularstructures. Thus, stem cells or progenitor cells can be recruited totarget specific sites, and corresponding cells and tissues are formed.

Following injection, the kidneys were evaluated at various time points(1, 2, 3, 4, 6, 9 and 12 weeks after injection) using histologicalstaining, H&E staining, and Masson's Trichrome staining. The followingimmunohistochemical stainings were used to evaluate the kidneysfollowing injection: BrdU cell tracing for detection of the recruitinghost cells; vascular endothelial cells (Factor VIII or CD31); Podocytes:Synaptopotin; Proximal tubular cells: Aquaporin 14; Tubular cells:Cytokeratin; Proliferating cell nuclear antigen (PCNA) staining; andStem/progenitor cell markers (CD34, CD44, CD45, CD90, CD105, CD133,Flk-1, Sca-1).

Histological images at 2 weeks after collagen hydrogel injectionidentified glomerular structures and tubular structures in the injuredarea: Likewise, histological images each week for 4 weeks afterkidney-derived hydrogel injection identified glomeruli in the injuredarea. FIG. 6 is a graph showing the numbers of glomeruli in the injuredarea at week 2 after hydrogel injection (based on ×100 magnification ofPCNA staining, *P<0.05, **area=0.57 mm²). Counting of glomerularstructure was done with anti-PCNA staining.

Example 9: Regeneration Potential in the Renal Disease Mouse Model Usingan Injectable Collagen Hydrogel

In the tissue regeneration study, injection of gel scaffolds into kidneyparenchyma resulted in multiple glomerular and tubular structureformation by 1 week and continued to mature with time. Presence of redblood cells was observed within the glomeruli, which is confined in theBowman's capsule. The cells consisting of the newly formed renalstructures expressed BrdU and proliferative cell nucleus antigen, whichindicate dividing and proliferating cells. These observations suggestthat the renal structures found within the injected site are regeneratedtissues. Glomerularendothelial and tubular structures were confirmedusing CD31 and Cytokeratin antibodies.

In this example, CBA/J inbred mice, an art recognized renal diseasemodel showing renal tubulointerstitial lesions, was used. CBA/J inbredmice are widely used as a general purpose strain. CBA/J strain is theonly CBA substrain that carries the Pde6b^(rdl) mutation, which causesblindness by wean age. The CBA/J inbred mouse strain is used to studygranulomatous experimental autoimmune thyroiditis (G-EAT), is relativelyresistant to diet-induced atherosclerosis, and develops a mild hearingloss late in life, with most of the hearing loss occurring in the higherfrequencies. Renal tubulointerstitial lesions have been observed in thisstrain at a high frequency. Some CBA/J mice spontaneously developexocrine pancreatic insufficiency syndrome with a high frequency ofrenal tubulointerstitial lesions in a disease mouse model (CBA/J mouse,Jackson Lab.).

Two experimental groups were compared (saline injection (0.9% sodiumchloride, Abbott Laboratories) control, and rat tail type I collagen(3.5 mg/mL, BD Biosciences)) at 1, 2, 3, and 4 weeks after injectionusing histological staining, H&E staining, and Masson's Trichromestaining. The following immunohistochemical staining was used toevaluate the injured area in the kidneys at 1, 2, and 4 weeks aftercollagen hydrogel following injection: BrdU cell tracing for detectionof the recruiting host cells; vascular endothelial cells (Factor VIII orCD31); Podocytes: Synaptopotin; Proximal tubular cells: Aquaporin 1/2;Tubular cells: Cytokeratin; Proliferating cell nuclear antigen (PCNA)staining; and stem/progenitor cell markers (CD34, CD44, CD45, CD90,CD105, CD133, Flk-1, Sca-1). FIG. 7 is a graph showing the number ofglomeruli in the injured area at week 2 after hydrogel injection (basedon ×100 magnification of PCNA staining, *P<0.05, **area=0.57 mm²).Collagen injection more than doubled the number of glomeruli compared tothe saline control. Counting of glomerular structure was done withanti-PCNA staining.

In addition, a rat renal insufficiency model was created to test whetherregeneration occurs in the diseased kidney. Lewis rats (male) were used.Renal insufficiency was induced by ligation of the renal artery and veinfor 60 min. Characterization was done (BUN/Cr and histology). Injectionof collagen into the pathologic kidney showed regeneration of kidneytissues as demonstrated by H&E staining, and immunohistochemistrystaining for CD44 and PCNA.

Example 10: Sustained Pressure Needed for Renal Tissue Regeneration

To determine whether regeneration of kidney structures is due to thecreated space, pressure or both, space was created surgically within therenal cortex without pressure. FIG. 8A shows a schematic of the CD1mouse model kidney. FIG. 8B shows a schematic of the space created inthe kidney by a 2 mm biopsy punch. A 2 mm biopsy punch was created in amouse kidney. Collagen injected into the surgically created space in therenal cortex showed no evidence of renal structure or cortex formation,indicating that pressure is necessary to initiate the regenerativeprocess. Provision of space without pressure showed the formation ofadipose-like tissues when analyzed by immunohistochemical staining inthe injured area at week 2 after formation of the surgically createdspace. FIG. 9 is a graph comparing the pressure within the kidneyfollowing injection of (a) 2 mg/ml collagen and (b) saline versus time.Table 1 shows the pressure measurement within the kidney afterinjection.

TABLE 1 Pressure measurement within the kidney following injection.Maximum Pressure Sustained Pressure (cmH₂O) (cmH₂O) Collagen 18.6 ± 3.313.3 ± 4.9 Saline 15.9 ± 1.5  0.5 ± 0.5* *Lewis Rats (n = 3), *P < 0.05

Example 11: Characterization of Infiltrating Host Cells and Renal TissueRegeneration in the Normal Mouse Kidney

Neutralized collagen gels (0.2% wt/vol) were injected into the kidneysof normal CD1 mice. Saline injections and needle sticks withoutinjection of material were also performed as controls. At 2 weeks afterinjection, each kidney contained inflammatory and fibroblastic cells inthe injected regions. However, in the collagen group, a higher number ofnewly formed glomerular-like structures was seen residing in theinjected collagen when compared to the other groups. In order to detecthost cells that had infiltrated into the injection regions of thekidney, we examined the localization of cells positive for PCNA, whichis expressed particularly in the early G1 and S phases of the cell cycleand is a marker for proliferating cells. Kidneys injected with collagencontained a large PCNA-positive cell population and showed progressiverenal tissue formation in the injected regions over time. The number ofPCNA-positive cells in the injected regions was significantly higherthan in normal regions (P<0.01, FIG. 10). This indicates that host cellscould migrate from other areas into the injected regions. Interestingly,it seems that these host cells (PCNA-positive) contribute to theformation of renal structures, especially the high number of glomerulithat were observed in the injected regions in collagen treated animals.

To determine if the cells that infiltrated the collagen biomaterial werehost renal stem/progenitor cells, immunohistochemistry for CD24, CD44,and CD 133 was performed. We found that a population of host cellsexpressing PAX-2, CD24, CD133 and CD44 was able to infiltrate theinjection regions of both normal mice and rats with renalischemia/reperfusion injury. The infiltrating host cells present withinthe injected regions of these kidneys expressed both renalstem/progenitor cell markers, CD24 and CD133, as well as the mesenchymalstem cell marker, CD44. We observed the presence of CD44⁺ cells in theinjected regions after collagen gel injection. The CD44⁺ cells werelocalized within the tubular area and at the glomerular level within theparietal layer of Bowman's capsule. However, we did not observe CD44+cells in the normal regions of the same kidney. In addition, theregenerated renal structures were identified by immunohistochemistrywith renal cell specific markers. The glomerular-like structureexpressed synaptopodin and CD31 and tubular-like structure expressedcytokeratin in a manner nearly identical to that of glomeruli andtubules in native kidney tissue. In fact, the number of glomeruli foundin the collagen gel regions was significantly higher compared to nativekidney tissue regions and the other groups (P<0.05, FIG. 11). Moreover,it seems that these cells contribute to the development of new renaltissue structure. These cells proliferate and eventuallyre-differentiate into typical renal cells during the regenerativeprocess.

Example 12: Renal Ischemia/Reperfusion Rat Model

All of the animals undergoing renal ischemia and reperfusion proceduressurvived at least 3 weeks after injury. At 2 weeks after the surgery,the kidney samples were retrieved and analyzed. The ischemic kidneysshowed tubular dilation and brush border loss as well as intratubularcast formation and degeneration of tubular architecture. Some tubularstructures became edematous and necrotic. The number of glomerulidecreased in the injured kidneys, and some swelled and developedsclerosis. After ischemia/reperfusion injury was confirmed, the collagengels were injected directly into the ischemic kidneys. The injectedregions could be easily identified, as most showed the presence ofincreased interstitial leukocytes and other infiltrating cells.

Newly formed glomerular-like structures and tubular-like structures wereobserved at 2 weeks after the collagen injection. PCNA-positive cells inthe injected regions were increased. To characterize the phenotype ofthe newly formed structures, we examined the expression of the renalcell markers, synaptopodin and CD31. In addition, the tubular structurestained positive for neprilysin.

To identify the host cells present in the injected regions,immunohistochemistry for the specific renal stem/progenitor cell markerswas performed. Immunohistochemistry of the collagen injection regionsfrom ischemic kidneys revealed that CD24, a marker of the renalembryonic progenitor cells, was expressed. In the injection regions, weobserved the presence of CD133⁺ and CD24⁺ cells of the interstitial andtubular structures. The CD44⁺ and PAX-2⁺ cells were identified not onlyin the interstitial and tubular structure, but also at the glomerularlevel within the parietal layer of Bowman's capsule in the injectionarea. These cells were rarely found in the native kidney tissue outsidethe injection regions.

After ischemic injury, the number of glomeruli in the native tissue wassignificantly decreased (7.9±0.35/mm², 4.02±0.18/mm², and3.29±0.186/mm²; normal kidney, 2 weeks, and 4 weeks after surgery,respectively; P<0.0001). However, in the collagen injection group, thenumber of glomeruli was significantly increased in the injected regionscompared to native regions with ischemic injury (11.44±0.72/mm² vs.4.02±0.18/mm²; 12.08±1.2/mm² vs. 3.29±0.186/mm²; 2 weeks and 4 weeks;P<0.01). Interestingly, the density of the glomeruli was higher withinthe collagen biomaterial than in normal kidney (FIGS. 12A-B).

In order to determine whether the newly formed glomeruli and tubulescould contribute to functional recovery of the kidney, blood sampleswere analyzed for creatinine levels. One week after the injections,renal function improved in all experimental groups (blood serumcreatinine level; 1.26±0.29 mg/dl in the sham; 0.93±0.16 mg/dl in thesaline group, and 0.76±0.1 mg/dl in the collagen group). Though therewas no significant difference in the three groups, there was a trendindicating that collagen injection led to the most improvement in renalfunction, while the sham group had the least (FIG. 13).

Example 13: Positive Pressure Measurement During the Injection andNumber of Glomeruli after Injection

The positive pressure in the injection regions of the kidney wasmeasured after saline and collagen injections. After saline injection,the pressure surged in seconds and returned to baseline quickly. In thecollagen groups, the pressure gradually increased to a peak thenmaintained over 70% of the ultimate pressure. The 2 mg/mL and 5 mg/mLconcentrations had higher peaks and sustained pressure as compared withsaline and 1 mg/mL collagen, while there was no difference between 2mg/mL and 5 mg/mL groups (FIGS. 14 and 15). We assume that the collagengels induce partial damage in the kidney as well as provide a mechanicalstimulus caused by positive pressure

With regards to a biopsy punch injury, we made a specific defectextended from cortex to medulla. At 2 weeks after injury, renalregeneration was observed in either collagen-filled or blank kidneys.The collagen group maintained the initial shape of the kidney, howeverthe control group collapsed. Histologically, numerous newly formedglomeruli were observed in the collagen group. In addition, the spaceprovided by the biopsy punch may accommodate the recruited cells andallow differentiation into required lineages to participate intissue/organ repair and regeneration.

Total number of glomeruli in the whole kidney section was decreasedafter ischemia injury (283.2±6.40 vs. 171.9±4.88, p<0.0001). After aninitial or multiple collagen injections, the total number of glomeruliincreased compared to ischemia-injured kidney (p=0.001). However, therewas no difference between 1st, 2nd, and 3rd injections (FIG. 16).

1. Use of a biocompatible substance as an implant to recruit pluripotentcells to a target site within a body structure and induce tissueregeneration within the body structure.
 2. The use according to claim 1,wherein the biocompatible substance creates a hyperbaric environment atthe target site such that native pluripotent cells migrate to the site.3. The use according to claim 1, wherein the biocompatible substancefurther provides a scaffold for attachment of pluripotent cells.
 4. Theuse according to claim 3, wherein the biocompatible substance furthercomprises a hydrogel that is sufficiently porous to allow cellinfiltration.
 5. The use according to claim 1, wherein the biocompatiblesubstance further comprises a hydrogel formulated to expand by waterabsorption following implantation at the target site to provide asustained positive pressure at the target site.
 6. The use according toclaim 1, wherein the biocompatible substance further comprises asolution having a collagen concentration from about 2 mg/mL to about 5mg/mL.
 7. The use according to claim 1, wherein the biocompatiblesubstance further comprises at least one adjuvant selected from thegroup of growth factors, cytokines, enzymes, collagen reducing agents,antibiotics and anti-inflammatory agents.
 8. The use according to claim7, wherein the adjuvant further comprises an anti-inflammatory agent. 9.The use according to claim 1, wherein the biocompatible substancefurther comprises a collage synthetase inhibitor.
 10. The use accordingto claim 1, wherein the biocompatible substance further comprises aninjectable composition.
 11. A method of inducing cell recruitment to atarget site of a subject comprising: obtaining a biocompatible substancehaving a viscosity greater than water; delivering the biocompatiblesubstance to a target site within a body structure to produce positivepressure within the target site; and maintaining positive pressure atthe target site for a period of time to create a space at the targetsite, whereby pluripotent cells are recruited to the target site. 12.The method of claim 11, wherein the step of delivering the biocompatiblesubstance further comprises delivering a substantially cell-freepolymeric biocompatible sub stance.
 13. The method of claim 11, whereinthe step of delivering the biocompatible substance further comprisesdelivering a collagen biocompatible substance.
 14. The method of claim13, wherein the collagen biocompatible substance is a solution having acollagen concentration from about 2 mg/mL to about 5 mg/mL.
 15. Themethod of claim 14, wherein the collagen solution thermogels at bodytemperature and the method further comprises delivering thebiocompatible substance by injection in a chilled state.
 16. The methodof claim 11, wherein the step of delivering the biocompatible substancefurther comprises delivering a hydrogel biocompatible substance.
 17. Themethod of claim 11, wherein the positive pressure is maintained in arange from about cmH₂O to about 70 cmH₂O for at least one hour afterinjection at the target site.
 18. The method of claim 11, wherein thestep of delivering the biocompatible substance further comprisesdelivering a biocompatible substance with a viscosity between about 5 cPto about 1×10⁸ cP at 25° C.
 19. The method of claim 11, wherein the stepof delivering the biocompatible substance into the target site comprisesinjecting the substance into the target site.
 20. The method of claim11, wherein the step of delivering the biocompatible substance into thetarget site comprises surgically implanting the substance at the targetsite.
 21. A method of kidney regeneration comprising: injecting asubstantially cell-free biocompatible substance into a target site in akidney, wherein the biocompatible substance creates a hyperbaricenvironment at the target site; maintaining positive pressure at thetarget site for at least one hour to recruit pluripotent cells to thetarget site; and inducing differentiation of the pluripotent cells,wherein differentiation produces new glomeruli structures within thekidney.
 22. A method of inducing in situ tissue regeneration at a targetsite of a subject comprising: obtaining a biocompatible thermogel thatwill thermogel as it reaches body temperature and maintain a viscositygreater than water for more than 1 hour when gelled, chilling thebiocompatible thermogel below its gelation temperature, delivering thechilled biocompatible substance to the target site to produce a positivepressure within a body structure, maintaining the positive pressure atthe target site for a period of time due to gelation of thebiocompatible thermogel as it reaches body temperature, wherebyprogenitor cells will be recruited and differentiate to regeneratetissue at the target site, and promoting differentiation of thepluripotent cells to regenerate the tissue at the target site.
 23. Themethod of claim 22, wherein the biocompatible substance comprises asubstantially cell-free, injectable biocompatible polymeric substance.24. The method of claim 22, wherein method further comprises deliveringat least one adjuvant to a target site.
 25. The method of claim 22,wherein the adjuvant further comprises an anti-inflammatory agent, suchas, for example, dexamethasone.
 26. The method of claim 22, wherein theadjuvant further comprises collagen synthetase inhibitor, such as, forexample, collagenase.
 27. The method according to claim 11, wherein thebiocompatible substance further comprises a collagen hydrogel that issufficiently porous to allow cell infiltration.
 28. The method accordingto claim 11, wherein the biocompatible substance further comprises acollagen hydrogel formulated to expand by water absorption followingimplantation at the target site to provide a sustained positive pressureat the target site.